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JeT
Trying to formalize a long question about (a kind) of validation of my workflow, I realized a good tree would actually summarize well my thoughts.
So... before asking for a proof of concept of this method to generate easily loads of content from one source, I'd like to formalize it visually (this question is a good pretext for me to formalize my thoughts and discover `forest`).

**My question**
How can I grow a second tree starting from courses, going left with a simple structure on 2 levels below courses like

* Courses
	- chap01
       	- sec01_01
       	- sec01_02
	- chap02
       	- sec02_01
       	- sec02_02
       	- sec02_03
	- ...
	- chap30
       	- sec30_01
       	- sec30_02
        
and having the same system of columns on the left hand side. The option grow'=180 enables me to switch correctly but I need on **both sides** (on the left, for chap and sec, on the right hand side of the tree as it is now) ?



Adapted from https://tex.stackexchange.com/questions/567434/tikz-tree-diagram-classification-with-functional-levels/567923#567923 I have the following (looong) tree...  (not so MWE but it'll be the key to my next post)

![TAQuestionTree.png](/image?hash=799a3adc6a6b73841c6ac84614a4f19b2362cab7581b12fe223fefa7e9987da9)

![TAQuestionTree0.png](/image?hash=6fc824a027aa0f87fb07622205412e711c61112f448656bb61d8d49408e4a2f1)

and the other side of the tree

![TAQuestionTree2.png](/image?hash=d890e4a3915dadb8455d8675f36d3850054018c1d42c4ec009c7363213c67ff0)

...that is the direct application of

**How to wrap a frame in a `tcolorbox` when using `\mode<article>`**

https://topanswers.xyz/tex?q=1089
(show frames in beamer ans tcolorboxes in beamerarticle)

**How to wrap `floats` in `tcolorbox` ?**

https://topanswers.xyz/tex?q=1411
(extend the previous to definition, examples, blocs... to tcolorboxes in beamerarticle)

**ignoreframe option (kind of) in Beamerarticle?**

https://topanswers.xyz/tex?q=1093

**Pass pgfkeys arguments to a document to produce multiple outputs**
https://topanswers.xyz/tex?q=1396
(to generate easily multiple outputs based on pgf keys)

**How to use pgfkeys to (elegantly) parameter my document**

https://topanswers.xyz/tex?q=1375

**Flag the lectures you want to compile for a course**
(smart filter of chapters/lectures to produce courses)
https://topanswers.xyz/tex?q=1093

**PS** : I have already fantastic results to produce with a simple `latexmk  ` consistent content between different formats that fit the needs of each audience (prof, student, administration, publication, printing, etc all `styles` using the combination of keys).

**PPS** : The same system of choice would be made on the left hand side to describe which chapters/lectures or sections to use to build the content. The formatting is taken care of by the styles on the right hand side.

**PPPS** : I am probably reinventing the wheel here, but the path was nice :)


```
\documentclass{standalone}
\usepackage[edges]{forest}

\forestset{
  % This is the style that should be applied in the tree preamble. See the tree
  % for what argument it takes.
  categorized citations/.style={
    delay={
      for tree={
        % The assumption is that all the nodes without a "tier" option belong
        % to the tree on the left. The names of the tiers that citation nodes
        % live on will end up to be consecutive numbers.
        if tier={}{
          % These styles can be adjusted by the user.
          common node options,
          tree node options,
        }{% We assume that anything on a tier is a citation node.
          citation format,
          % Split the content of the node (separator is ",") and format the
          % parts --- if desired.
          if citation node autoformat={
            split option={content}{,}{format first citation entry, format citation entry},
          }{},
          % The tier situation is atypical in these trees. There are multiple
          % tiers, but there is no hierarchical relation between them --- all
          % nodes on tiers are just children of the nodes in the tree.  So we
          % need to give forest a nudge to get tiers right. This has two parts.
          % 
          % Tiers, part 1. Here we solve the problem of a node in the
          % tree with several tier nodes (i.e. citations).  The citation nodes
          % are siblings, so forest will push them apart in the s (y) dimension
          % --- this is so because tier alignment happens later.  But if we
          % make the l (x) coordinates of the nodes different enough, the nodes
          % will not "bump" and will happily end up with the same s (y) coordinate.
          % We try to make them minimally different (and thus small), so that
          % we don't accidentally push them too much to the right.
          l=\forestregister{citation boxes offset}+(tier()-1)*(\forestregister{citation node text width}+2*\pgfkeysvalueof{/pgf/inner xsep}+2*\pgfkeysvalueof{/pgf/outer xsep}+0.1pt),
        },
      },
      % Tiers, part 2.
      % Here we tell forest about the relative order of tiers (and thus boxes
      % on the right). We need to do this because when we write down the tree,
      % we just put the citation nodes next to each other (as siblings).  The
      % idea here is to create a dummy node on each tier, with structure
      % [1[2[3...]]], and remove them all once they do their job. This "tier
      % header" is created automatically from the argument to "folders with
      % citation blocks" style.
      temptoksa={}, % This will hold the argument to "prepend".
      % This will hold a call of "draw tier box" for each tier box (in reverse
      % order):
      draw tier boxes/.style={},
      tempcounta'=0, % The current tier number.
      temptoksb={}, % The closing brackets to append to temptoksa.
      split={#1}{,}{add first citation tier, add citation tier},
      % Insert the closing brackets.
      temptoksa+/.register=temptoksb,
      % The tier header specification is constructed, let's put it in the tree!
      prepend/.register=temptoksa,
    },
    % After all nodes are positioned, we define style "draw tier box" which
    % draws a citation box and its contents.
    before drawing tree={% #1 = tier name
      % Remember the topmost and the bottommost position in the entire tree
      % (folders and citations):
      tempdimya/.max={y()+max_y()}{tree},
      tempdimyb/.min={y()+min_y()}{tree},
      % The first argument of this style is a tier name; in the code, it is
      % referred to by ####1, due to being embedded in .process
      draw tier box/.style/.process=R2w2{tempdimya}{tempdimyb}{
        % ##1, ##2 = tempdimya, tempdimyb
        % Get the leftmost and the right position in the citation box (i.e the
        % given tier):
        tempdimxa/.max={x()+max_x()}{filter={tree}{strequal(tier(),"####1")}},
        tempdimxb/.min={x()+min_x()}{filter={tree}{strequal(tier(),"####1")}},
        % "draw tier box" style is called from the "draw tree method", so we
        % just draw the citation box directly.
        TeX/.process=R2ORw4
          {tempdimxa}{tempdimxb}{####1.content}{citation box label position}{
          % ########1, ########2 = tempdimxa, tempdimxb
          % ########3 = the content of the dummy tier node
          % ########4 = "citation box label position" register
          \node[
            fit={(########1,##1) (########2,##2)},
            citation box options,
            % The box is labeled by the content of the dummy tier header node.
            label={[citation box label options]########4:########3}]{};
        },
        % Draw the citation nodes of this box (i.e. on this tier), and their edges.
        for filter={tree}{strequal(tier(),"####1")}{draw tree node, draw tree edge},
      },
    },
    % We need to draw stuff in a very particular order, so that the edges from
    % citation nodes on tier 2 go behind citation box 1, etc.
    draw tree method/.style={
      % Draw the tree on the left (nodes and edges).
      for filter={tree}{strequal(tier(),"")}{draw tree node, draw tree edge},
      % Draw the citation boxes (and their contents) --- in reverse order!
      draw tier boxes,
      % Finally, draw any decorations.
      for tree=draw tree tikz,
    },
  },
  % How should the nodes in the blocks on the right look like?
  citation format/.style={
    % Don't change "text width" directly. We need to remember it for later (see
    % Tiers, part 1), so use "citation node text width" register.
    text width/.register=citation node text width,
    % Adjust these styles for other options.
    common node options,
    citation node options,
  },
  % We use these two styles when splitting the citation box specification (the
  % argument of "categorized citations").
  add first citation tier/.style={
    add citation tier={#1},
    % Get rid of the dummy citation nodes after they have done their job.
    temptoksa+={, before computing xy=remove},
  },
  add citation tier/.style={
    tempcounta'+=1, % Increase the tier number.
    % Add the dummy citation specification. We put it on tier 1/2/3..., give it
    % the same name (just in case we want to format them independently), and
    % apply the standard citation node format.
    temptoksa+/.process=Rw{tempcounta}{[#1, tier=##1, name=##1, citation format%]
      },
    % Well need an extra closing bracket at the end:
    temptoksb+=%[
    ],
    % Prepend the command to draw this particular citation box to "draw tier
    % boxes".  We *pre*pend because it is crucial that citation boxes are drawn
    % in reverse order: the idea is that a citation box gets drawn on top of
    % any edges coming from citation boxes further away from the tree; in
    % effect, the edges to citations will go below intermediate citation boxes.
    draw tier boxes/.prefix style/.process=Rw{tempcounta}{draw tier box=##1},
  },
  % An update to the "folder" style from the "edges" library. It (or something
  % like it) will be included in the next release of forest.
  folder v2/.style={
    calign=child,
    calign primary child=1,
    % This is necessary for "tempdims" calculation below to work properly.
    anchor=parent,
    after packing node={
      if n children=0{}{
        tempdiml=l_sep()-l("!1"),  % l-shift
        tempdims={abs(max_s("",""))+abs(min_s("!1",""))+s_sep()}, % s-shift
        for children={
          l+=tempdiml,
          s+=tempdims()*(0.5-reversed())*2,
          edge={rotate/.option=!parent.grow},
          % We don't use the values of "parent anchor" and "child anchor" here
          % (we use ".-children last" and ".parent" directly), because the user
          % might want to use those otherwise if the folder is embedded in a
          % larger tree.
          edge path'/.expanded={
            ([xshift=\forestregister{folder indent}]!u.-children last) |- (.parent)
          },
        },
        fit=band, % to avoid overlapping nodes with their (greatgreat...)uncles.
      },
    },
  },
  % In "folder with cites", we separate category and citation nodes, and pack
  % them separately. So this style really does all the work in "before/after
  % packing node": separate category and citation nodes just before packing, and
  % put them back together after packing.
  % 
  % This is just a temporary node we'll need to store a copy of the parent
  % folder node into. We need it because somebody didn't implement "create" to
  % accept a relative node name and thus act as a copying operation. To be done.-
  create'={[,name=folder@temp]},
  folder with cites/.style={
    before packing node={
      % Make a copy of the parent folder node (without the subtree).
      for name/.process=_Ow{folder@temp}{id}{
        append''={!{id=##1}},
      },
      % Here I don't follow my own advice (from the forest manual) and call the
      % experimental "do dynamics" from within "process keylist(')" (well, not
      % explicitly, effectively so, because "before/after packing node" are
      % internally called as if by "process keylist'"). The bad news is that
      % "do dynamics" in fact does not work as expected here ...
      do dynamics,
      % ... but the good news is that I now know at least one thing that is
      % wrong with it: it fails to set "last dynamic node". This will be
      % investigated ... until then, a workaround:
      for group={name=folder@temp,last}{alias=folder@temp@parent},
      % In fact, there was another problem with "do dynamics". It did not
      % update all the node options containing the hierarchical information
      % about the tree. So this key now grew (the new definition below; to be
      % included in the next release of forest) an argument: a relative node
      % name instructing it which nodes to update after doing the dynamic
      % thing. (Note that we can safely use "do dynamics" twice; no dynamic
      % operations are performed the second time, as the dynamic queue is
      % empty, but the info will get updated.)
      do dynamics=folder@temp@parent,
      % Move all citation nodes (i.e. a nodes with non-empty tier option) into
      % the copy of the parent.
      for children={
        if tier={}{}{
          for name/.process=_Ow{folder@temp@parent}{id}{
            append={!{id=##1}},
          },
        },
      },
      % Do the dynamic operations immediately --- ahh yes, we must do this
      % because we're in the middle of packing a node!
      do dynamics, do dynamics=folder@temp@parent,
    },
    % Apply the (updated) folder style to whatever children remained in the
    % original parent. 
    folder v2,
    after packing node={
      alias=folder@temp@current,
      for name={folder@temp@parent}{
        for children={
          % We do this because gdjgfjdfgsdj ... remove the line and see what
          % happens to the tall adjacent citations on tier 2 (children of
          % "Crank and slider" and "Bell crank").
          l+={abs(max_l("",""))+abs(min_l("!u",""))+l_sep("!u")},
        },
        % calign/.register=citation nodes calign,
        % How should we calign the citation nodes after packing them?
        citation nodes calign,
        % Pack the citation nodes within the copy of the parent ...
        pack',
        % ... and then put them back into the original parent. It does not
        % really matter where to put them, so we just append.
        for children={
          for name/.process=_Ow{folder@temp@current}{id}{
            append={!{id=##1}},
          },
        },
      },
      % For the final time ...
      do dynamics,
    },
  },
  declare dimen register=citation node text width,
  declare dimen register=citation boxes offset,
  declare boolean register=citation node autoformat,
  declare toks register=citation box label position,
}

\makeatletter
\forestset{
  % Let's patch up "do dynamics" --- to be included in the next release of forest.
  do dynamics/.code={%
    \the\forest@do@dynamics
    \forest@do@dynamics{}%
    \forest@forthis{%
      \forest@nameandgo{#1}%
      \forest@node@Compute@numeric@ts@info{\forest@cn}%
    }%
  },
  do dynamics/.default=!{root'},
}
\makeatother


% These are the formatting options and should be (reasonably) safe to adjust.
\forestset{
  common node options/.style={
    % "grow" will only work for 0 and 180. For other directions, the "draw tier
    % box" style would need to be generalized.
    grow'=0,
  },
  % This style is applied to all the category nodes, i.e. the nodes on the
  % left.
  tree node options/.style={
    % Some basic stuff ...
    draw, /tikz/align=center,
    % For single citation nodes connected to a tree nodes, it even works
    % without this (assuming that the default parent anchor is center). But for
    % the fancy "forked edge" calignment of citation nodes, this is necessary.
    parent anchor=children,
    % The shift of the parent anchor for the folder node (a register applying
    % to all folder nodes):
    folder indent=1em,
    % Nodes on levels 0, 1 and 2 are drawn with "forked edge"s.
    % Nodes on levels 2, 3, ... are folders. There is no limit on the
    % number of levels.
    % 
    % Feel free to change the level where the transition from forked edge to
    % folders occurs, or even to mix them. Note that key "forked edge" sets the
    % type of the edge towards the parent node; it must be set for each child.  Key
    % "folder" (either the original, v2, or "folder with cites"), on the other hand,
    % influences the positions and edges of its children; it must be only used on
    % the parent node.
    % 
    % We also set text widths, opacities etc. for each level here. The
    % color is set in the tree itself.
    if level=0{
      fill opacity=0.45, text width=3.5cm, rounded corners=3pt,
    }{if level=1{
        fill opacity=0.45, text width=2.5cm, rounded corners=3pt,
        forked edge,
      }{
        if level=2{
          fill opacity=0.30, text width=2.0cm, rounded corners=3pt,
          forked edge,
          % The "l sep" (here and below) influences both the folder--file
          % distance and the distance to citation nodes.
          folder with cites, l sep+=1em,
        }{
          if level=3{
            fill opacity=0.30, text width=1.5cm, rounded corners=2pt,
            folder with cites, l sep+=1em,
          }{% level >= 4
            fill opacity=0.15, text width=1.5cm, rounded corners=2pt,
            folder with cites, l sep+=1em,
          },
        },
      },
    },
  },
  % The width of the citation nodes.
  citation node text width=1.5cm,
  % Other options applying to citation nodes (don't change "text width" here!):
  citation node options/.style={
    draw, /tikz/align=center, rounded corners=2pt,
    fill=brown, fill opacity=0.6,
    % This sets the separation between the citation boxes:
    l sep=2em,
    % The "anchor" key specifies center vertical alignment to the parent.  The
    % "child anchor" says that's where the edge will start too. We
    % most probably want to keep these as they are.
    anchor=parent, child anchor=parent,
  },
  % How shall we align citations in the unlikely case that we have more than
  % one citation node belonging to a category node on a single tier (see the blue
  % "citation" in the example tree)? Let's have a very fancy setup, center
  % caligned with forked edges, by default.
  citation nodes calign/.style={
    calign=center, forked edges, for children={fork sep=2em},
  },
  % We can push the citation boxes a bit further away from the tree. By
  % default, the "l sep" of the parents of citations nodes is in effect.
  citation boxes offset=0em,
  % How should a citation box look like? Note that we want "fill opacity=1"
  % (the default) here, because we want the citation box to partially hide the
  % edges from citations to the folders.
  /tikz/citation box options/.style={
    fill=blue!20, draw=red, thick,
  },
  % Format the citation box labels:
  /tikz/citation box label options/.style={
  },
  % Where should the citation box labels appear?
  citation box label position=north,
  % These two keys are used to automatically format the list of references in a
  % citation node.
  format first citation entry/.style={content'=\mbox{[#1]}},
  format citation entry/.style={content+'=\discretionary{}{}{}\mbox{[#1]}},
  % A register saying whether we want to autoformat citations:
  citation node autoformat=true,
}

\begin{document}
\begin{forest}
 % Style "categorized citations" takes an argument specifying the
 % (comma-separated) labels of citation boxes.  Each citation block will reside
 % on its own tier, the tiers numbered 1,2,3... So to put a citation into
 % block A/B/C, write "tier=1/2/3" into the citation node.  The number of
 % labels given here must match the number of tiers used below, i.e. an empty
 % citation box will lead to an error.  If you list too few labels here, the
 % extra tiers will be ignored (without producing an error).
 categorized citations={standard,prof,student}
 [Courses, fill=gray
  [common, for tree={fill=blue}
   [class
    [book]
    [beamer]
    [article]
    [exam]
    [...]
   ]
   [Author related
    [author]
    [title]
    [subtitle]
   ] 
   [cover-image
    [cover Course1]
    [cover Course2]
    [cover Course3]
    [cover Course4]
    [...]
    [error-handler]
    [tikzexternalize
     [yes
      [,tier=1]
      [,tier=2]
     ]
     [no
      [,tier=3]
     ]
    ]       
   ]
   [University
    [institute]
    [department]
    [logo
     [standard]
     [Transparent]
     [BW]
    ]
   ]
  ]
  [notes, for tree={fill=brown}
   [page-format
    [standard
     [,tier=1]
    ]
    [wide
     [,tier=2]
    ]
    [tufte like
     [,tier=3]
    ]
   ]      
   [header-format
    [simple
     [,tier=1]
    ]
    [fancy
     [,tier=2]
     [,tier=3]
    ]
   ]  
   [fonts
    [Garamond
     [,tier=1]
    ]
    [Euler]
    [Times]
    [Avant
     [,tier=2]
     [,tier=3]
    ]
   ]      
   [caption]      
   [float-counters]      
   [frames in tcb
    [yes
     [,tier=1]
     [,tier=2]
    ]
    [no
     [,tier=3]
    ]
   ]      
   [env in tcb
    [example
     [,tier=1]
     [,tier=2]
    ]
    [definition
     [,tier=1]
     [,tier=2]
    ]
    [block
     [,tier=1]
     [,tier=2]
    ]
    [alertblock
     [,tier=1]
     [,tier=2]
    ]
   ]      
   [multipage
    [2 on 1
     [,tier=2]
    ]
    [4 on 1]
    [block]
   ]      
   [tocdepth
    [4,tier=1]
    [4,tier=2]
    [2,tier=3]
   ]      
   [section depth
    [4,tier=1]
    [4,tier=2]
    [3,tier=3]
   ]      
   [extract
    [yes
     [,tier=1]
     [,tier=2]
    ]
    [no
     [,tier=3]
    ]
   ]      
  ]
  [beamer,
   % An easy way to set the color of the entire subtree. The opacity is set,
   % per-level, in "tree node options".
   for tree={fill=red}, 
   [theme
    [Simple
     [,tier=1]
    ]
    [Cambridge
     [,tier=2]
    ]
    [Hannover
     [,tier=3]
    ]
   ]
   [colortheme
    [no color
     [,tier=1]
    ]
    [beetle
     [,tier=2]
    ]
    [crane
     [,tier=3]
    ]
   ]
   [font
    [,tier=1]
    [,tier=2]
    [professionalfonts,tier=3]
    [page-format
     [normal
      [,tier=1]
     ]      
     [show only notes
      [,tier=2]
     ]      
     [show notes on second screen=right
      [,tier=3]
     ]
    ]
   ]
   [university colors
   ]
   [tocdepth
    [2,tier=1]
    [2,tier=2]
    [2,tier=3]
   ]
  ]
  % 
 ]
\end{forest}
\end{document}
```

as for my poor attempt to describe left tree

```
\documentclass{standalone}
\usepackage[edges]{forest}

\forestset{
  % This is the style that should be applied in the tree preamble. See the tree
  % for what argument it takes.
  categorized citations/.style={
    delay={
      for tree={
        % The assumption is that all the nodes without a "tier" option belong
        % to the tree on the left. The names of the tiers that citation nodes
        % live on will end up to be consecutive numbers.
        if tier={}{
          % These styles can be adjusted by the user.
          common node options,
          tree node options,
        }{% We assume that anything on a tier is a citation node.
          citation format,
          % Split the content of the node (separator is ",") and format the
          % parts --- if desired.
          if citation node autoformat={
            split option={content}{,}{format first citation entry, format citation entry},
          }{},
          % The tier situation is atypical in these trees. There are multiple
          % tiers, but there is no hierarchical relation between them --- all
          % nodes on tiers are just children of the nodes in the tree.  So we
          % need to give forest a nudge to get tiers right. This has two parts.
          % 
          % Tiers, part 1. Here we solve the problem of a node in the
          % tree with several tier nodes (i.e. citations).  The citation nodes
          % are siblings, so forest will push them apart in the s (y) dimension
          % --- this is so because tier alignment happens later.  But if we
          % make the l (x) coordinates of the nodes different enough, the nodes
          % will not "bump" and will happily end up with the same s (y) coordinate.
          % We try to make them minimally different (and thus small), so that
          % we don't accidentally push them too much to the right.
          l=\forestregister{citation boxes offset}+(tier()-1)*(\forestregister{citation node text width}+2*\pgfkeysvalueof{/pgf/inner xsep}+2*\pgfkeysvalueof{/pgf/outer xsep}+0.1pt),
        },
      },
      % Tiers, part 2.
      % Here we tell forest about the relative order of tiers (and thus boxes
      % on the right). We need to do this because when we write down the tree,
      % we just put the citation nodes next to each other (as siblings).  The
      % idea here is to create a dummy node on each tier, with structure
      % [1[2[3...]]], and remove them all once they do their job. This "tier
      % header" is created automatically from the argument to "folders with
      % citation blocks" style.
      temptoksa={}, % This will hold the argument to "prepend".
      % This will hold a call of "draw tier box" for each tier box (in reverse
      % order):
      draw tier boxes/.style={},
      tempcounta'=0, % The current tier number.
      temptoksb={}, % The closing brackets to append to temptoksa.
      split={#1}{,}{add first citation tier, add citation tier},
      % Insert the closing brackets.
      temptoksa+/.register=temptoksb,
      % The tier header specification is constructed, let's put it in the tree!
      prepend/.register=temptoksa,
    },
    % After all nodes are positioned, we define style "draw tier box" which
    % draws a citation box and its contents.
    before drawing tree={% #1 = tier name
      % Remember the topmost and the bottommost position in the entire tree
      % (folders and citations):
      tempdimya/.max={y()+max_y()}{tree},
      tempdimyb/.min={y()+min_y()}{tree},
      % The first argument of this style is a tier name; in the code, it is
      % referred to by ####1, due to being embedded in .process
      draw tier box/.style/.process=R2w2{tempdimya}{tempdimyb}{
        % ##1, ##2 = tempdimya, tempdimyb
        % Get the leftmost and the right position in the citation box (i.e the
        % given tier):
        tempdimxa/.max={x()+max_x()}{filter={tree}{strequal(tier(),"####1")}},
        tempdimxb/.min={x()+min_x()}{filter={tree}{strequal(tier(),"####1")}},
        % "draw tier box" style is called from the "draw tree method", so we
        % just draw the citation box directly.
        TeX/.process=R2ORw4
          {tempdimxa}{tempdimxb}{####1.content}{citation box label position}{
          % ########1, ########2 = tempdimxa, tempdimxb
          % ########3 = the content of the dummy tier node
          % ########4 = "citation box label position" register
          \node[
            fit={(########1,##1) (########2,##2)},
            citation box options,
            % The box is labeled by the content of the dummy tier header node.
            label={[citation box label options]########4:########3}]{};
        },
        % Draw the citation nodes of this box (i.e. on this tier), and their edges.
        for filter={tree}{strequal(tier(),"####1")}{draw tree node, draw tree edge},
      },
    },
    % We need to draw stuff in a very particular order, so that the edges from
    % citation nodes on tier 2 go behind citation box 1, etc.
    draw tree method/.style={
      % Draw the tree on the left (nodes and edges).
      for filter={tree}{strequal(tier(),"")}{draw tree node, draw tree edge},
      % Draw the citation boxes (and their contents) --- in reverse order!
      draw tier boxes,
      % Finally, draw any decorations.
      for tree=draw tree tikz,
    },
  },
  % How should the nodes in the blocks on the right look like?
  citation format/.style={
    % Don't change "text width" directly. We need to remember it for later (see
    % Tiers, part 1), so use "citation node text width" register.
    text width/.register=citation node text width,
    % Adjust these styles for other options.
    common node options,
    citation node options,
  },
  % We use these two styles when splitting the citation box specification (the
  % argument of "categorized citations").
  add first citation tier/.style={
    add citation tier={#1},
    % Get rid of the dummy citation nodes after they have done their job.
    temptoksa+={, before computing xy=remove},
  },
  add citation tier/.style={
    tempcounta'+=1, % Increase the tier number.
    % Add the dummy citation specification. We put it on tier 1/2/3..., give it
    % the same name (just in case we want to format them independently), and
    % apply the standard citation node format.
    temptoksa+/.process=Rw{tempcounta}{[#1, tier=##1, name=##1, citation format%]
      },
    % Well need an extra closing bracket at the end:
    temptoksb+=%[
    ],
    % Prepend the command to draw this particular citation box to "draw tier
    % boxes".  We *pre*pend because it is crucial that citation boxes are drawn
    % in reverse order: the idea is that a citation box gets drawn on top of
    % any edges coming from citation boxes further away from the tree; in
    % effect, the edges to citations will go below intermediate citation boxes.
    draw tier boxes/.prefix style/.process=Rw{tempcounta}{draw tier box=##1},
  },
  % An update to the "folder" style from the "edges" library. It (or something
  % like it) will be included in the next release of forest.
  folder v2/.style={
    calign=child,
    calign primary child=1,
    % This is necessary for "tempdims" calculation below to work properly.
    anchor=parent,
    after packing node={
      if n children=0{}{
        tempdiml=l_sep()-l("!1"),  % l-shift
        tempdims={abs(max_s("",""))+abs(min_s("!1",""))+s_sep()}, % s-shift
        for children={
          l+=tempdiml,
          s+=tempdims()*(0.5-reversed())*2,
          edge={rotate/.option=!parent.grow},
          % We don't use the values of "parent anchor" and "child anchor" here
          % (we use ".-children last" and ".parent" directly), because the user
          % might want to use those otherwise if the folder is embedded in a
          % larger tree.
          edge path'/.expanded={
            ([xshift=\forestregister{folder indent}]!u.-children last) |- (.parent)
          },
        },
        fit=band, % to avoid overlapping nodes with their (greatgreat...)uncles.
      },
    },
  },
  % In "folder with cites", we separate category and citation nodes, and pack
  % them separately. So this style really does all the work in "before/after
  % packing node": separate category and citation nodes just before packing, and
  % put them back together after packing.
  % 
  % This is just a temporary node we'll need to store a copy of the parent
  % folder node into. We need it because somebody didn't implement "create" to
  % accept a relative node name and thus act as a copying operation. To be done.-
  create'={[,name=folder@temp]},
  folder with cites/.style={
    before packing node={
      % Make a copy of the parent folder node (without the subtree).
      for name/.process=_Ow{folder@temp}{id}{
        append''={!{id=##1}},
      },
      % Here I don't follow my own advice (from the forest manual) and call the
      % experimental "do dynamics" from within "process keylist(')" (well, not
      % explicitly, effectively so, because "before/after packing node" are
      % internally called as if by "process keylist'"). The bad news is that
      % "do dynamics" in fact does not work as expected here ...
      do dynamics,
      % ... but the good news is that I now know at least one thing that is
      % wrong with it: it fails to set "last dynamic node". This will be
      % investigated ... until then, a workaround:
      for group={name=folder@temp,last}{alias=folder@temp@parent},
      % In fact, there was another problem with "do dynamics". It did not
      % update all the node options containing the hierarchical information
      % about the tree. So this key now grew (the new definition below; to be
      % included in the next release of forest) an argument: a relative node
      % name instructing it which nodes to update after doing the dynamic
      % thing. (Note that we can safely use "do dynamics" twice; no dynamic
      % operations are performed the second time, as the dynamic queue is
      % empty, but the info will get updated.)
      do dynamics=folder@temp@parent,
      % Move all citation nodes (i.e. a nodes with non-empty tier option) into
      % the copy of the parent.
      for children={
        if tier={}{}{
          for name/.process=_Ow{folder@temp@parent}{id}{
            append={!{id=##1}},
          },
        },
      },
      % Do the dynamic operations immediately --- ahh yes, we must do this
      % because we're in the middle of packing a node!
      do dynamics, do dynamics=folder@temp@parent,
    },
    % Apply the (updated) folder style to whatever children remained in the
    % original parent. 
    folder v2,
    after packing node={
      alias=folder@temp@current,
      for name={folder@temp@parent}{
        for children={
          % We do this because gdjgfjdfgsdj ... remove the line and see what
          % happens to the tall adjacent citations on tier 2 (children of
          % "Crank and slider" and "Bell crank").
          l+={abs(max_l("",""))+abs(min_l("!u",""))+l_sep("!u")},
        },
        % calign/.register=citation nodes calign,
        % How should we calign the citation nodes after packing them?
        citation nodes calign,
        % Pack the citation nodes within the copy of the parent ...
        pack',
        % ... and then put them back into the original parent. It does not
        % really matter where to put them, so we just append.
        for children={
          for name/.process=_Ow{folder@temp@current}{id}{
            append={!{id=##1}},
          },
        },
      },
      % For the final time ...
      do dynamics,
    },
  },
  declare dimen register=citation node text width,
  declare dimen register=citation boxes offset,
  declare boolean register=citation node autoformat,
  declare toks register=citation box label position,
}

\makeatletter
\forestset{
  % Let's patch up "do dynamics" --- to be included in the next release of forest.
  do dynamics/.code={%
    \the\forest@do@dynamics
    \forest@do@dynamics{}%
    \forest@forthis{%
      \forest@nameandgo{#1}%
      \forest@node@Compute@numeric@ts@info{\forest@cn}%
    }%
  },
  do dynamics/.default=!{root'},
}
\makeatother


% These are the formatting options and should be (reasonably) safe to adjust.
\forestset{
  common node options/.style={
    % "grow" will only work for 0 and 180. For other directions, the "draw tier
    % box" style would need to be generalized.
    grow'=180,
  },
  % This style is applied to all the category nodes, i.e. the nodes on the
  % left.
  tree node options/.style={
    % Some basic stuff ...
    draw, /tikz/align=center,
    % For single citation nodes connected to a tree nodes, it even works
    % without this (assuming that the default parent anchor is center). But for
    % the fancy "forked edge" calignment of citation nodes, this is necessary.
    parent anchor=children,
    % The shift of the parent anchor for the folder node (a register applying
    % to all folder nodes):
    folder indent=1em,
    % Nodes on levels 0, 1 and 2 are drawn with "forked edge"s.
    % Nodes on levels 2, 3, ... are folders. There is no limit on the
    % number of levels.
    % 
    % Feel free to change the level where the transition from forked edge to
    % folders occurs, or even to mix them. Note that key "forked edge" sets the
    % type of the edge towards the parent node; it must be set for each child.  Key
    % "folder" (either the original, v2, or "folder with cites"), on the other hand,
    % influences the positions and edges of its children; it must be only used on
    % the parent node.
    % 
    % We also set text widths, opacities etc. for each level here. The
    % color is set in the tree itself.
    if level=0{
      fill opacity=0.45, text width=3.5cm, rounded corners=3pt,
    }{if level=1{
        fill opacity=0.45, text width=2.5cm, rounded corners=3pt,
        forked edge,
      }{
        if level=2{
          fill opacity=0.30, text width=2.0cm, rounded corners=3pt,
          forked edge,
          % The "l sep" (here and below) influences both the folder--file
          % distance and the distance to citation nodes.
          folder with cites, l sep+=1em,
        }{
          if level=3{
            fill opacity=0.30, text width=1.5cm, rounded corners=2pt,
            folder with cites, l sep+=1em,
          }{% level >= 4
            fill opacity=0.15, text width=1.5cm, rounded corners=2pt,
            folder with cites, l sep+=1em,
          },
        },
      },
    },
  },
  % The width of the citation nodes.
  citation node text width=1.5cm,
  % Other options applying to citation nodes (don't change "text width" here!):
  citation node options/.style={
    draw, /tikz/align=center, rounded corners=2pt,
    fill=brown, fill opacity=0.6,
    % This sets the separation between the citation boxes:
    l sep=2em,
    % The "anchor" key specifies center vertical alignment to the parent.  The
    % "child anchor" says that's where the edge will start too. We
    % most probably want to keep these as they are.
    anchor=parent, child anchor=parent,
  },
  % How shall we align citations in the unlikely case that we have more than
  % one citation node belonging to a category node on a single tier (see the blue
  % "citation" in the example tree)? Let's have a very fancy setup, center
  % caligned with forked edges, by default.
  citation nodes calign/.style={
    calign=center, forked edges, for children={fork sep=2em},
  },
  % We can push the citation boxes a bit further away from the tree. By
  % default, the "l sep" of the parents of citations nodes is in effect.
  citation boxes offset=0em,
  % How should a citation box look like? Note that we want "fill opacity=1"
  % (the default) here, because we want the citation box to partially hide the
  % edges from citations to the folders.
  /tikz/citation box options/.style={
    fill=blue!20, draw=red, thick,
  },
  % Format the citation box labels:
  /tikz/citation box label options/.style={
  },
  % Where should the citation box labels appear?
  citation box label position=north,
  % These two keys are used to automatically format the list of references in a
  % citation node.
  format first citation entry/.style={content'=\mbox{[#1]}},
  format citation entry/.style={content+'=\discretionary{}{}{}\mbox{[#1]}},
  % A register saying whether we want to autoformat citations:
  citation node autoformat=true,
}

\begin{document}
\begin{forest}
 % Style "categorized citations" takes an argument specifying the
 % (comma-separated) labels of citation boxes.  Each citation block will reside
 % on its own tier, the tiers numbered 1,2,3... So to put a citation into
 % block A/B/C, write "tier=1/2/3" into the citation node.  The number of
 % labels given here must match the number of tiers used below, i.e. an empty
 % citation box will lead to an error.  If you list too few labels here, the
 % extra tiers will be ignored (without producing an error).
 categorized citations={chapters/lectures,sections,}
 [Courses, fill=gray
  [chap01, for tree={fill=blue}
   [sec01-01]
   [sec01-02]
   [...]
   [sec01-n]   
  ]
  [chap02, for tree={fill=blue}
   [sec02-01]
   [sec02-02]
   [...]
   [sec02-n]   
  ]
  [chap03, for tree={fill=blue}
   [sec03-01]
   [sec03-02]
   [...]
   [sec03-n]   
  ]  
  [...]  
  [chap30,for tree={fill=red}, 
   [sec30-01
    [,tier=1]
    [sec30-02
     [,tier=2]
    ]
    [sec03-03
     [,tier=2]
    ]
   ]
  ]
 ]
\end{forest}
\end{document}
```
Top Answer
marmot
This is not really an answer, just some simpleminded approach. One can store one branch in a savebox. And one can measure the x and y coordinates of the root in this saved tree. This allows us to postion the root precisely on top of the root of another tree that grows in the opposite direction. 
```
\documentclass{standalone}
\usepackage[edges]{forest}
\newsavebox\ForestR
\sbox\ForestR{\begin{forest}
forked edges,
for tree={grow'=0,draw=red,dashed,rounded corners}
 [R,alias=R
  [A
   [1]
   [2]
   [3]
  ]
  [B
   [1]
   [2]
  ]
  [C
   [1]
   [2]
  ]
  [D
   [1]
   [2]  
  ]
 ]
\path[overlay] let \p1=($(current bounding box.north west)-(R.north west)$)
 in \pgfextra{\xdef\JeTy{\y1}\xdef\JeTx{\x1}};
\end{forest}}
\begin{document}
\begin{forest}
forked edges,
for tree={grow'=180,draw,rounded corners}
 [R,alias=R'
  [A
   [1]
   [2]
  ]
  [B
   [1]
   [2]
   [3]
  ]
  [C
   [1]
   [2]
   [3]
  ]
  [D
   [1]
   [2]  
  ]
 ]
\path (R'.north west) node[anchor=north west,
	xshift=-\JeTx-\pgfkeysvalueof{/pgf/inner xsep}-\pgfkeysvalueof{/pgf/outer xsep},	
	yshift=\JeTy+\pgfkeysvalueof{/pgf/inner ysep}+\pgfkeysvalueof{/pgf/outer ysep}
	]{\usebox\ForestR};
\end{forest}
\end{document}
```
![Screen Shot 2020-10-22 at 9.54.59 PM.png](/image?hash=0a3a2d88f34a06507e9e2063bd3101ba338ba57d808e9c19df7bdb3ecf1a6b98)

I used red dashes to show that the roots really end up at the same positions.

![Screen Shot 2020-10-22 at 9.55.22 PM.png](/image?hash=fbfab8bac2fd9559a06c5402c7d4af1b3901f862ff57cae53e69ac091d408b6e)

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