This is my stripped down LaTeX code. There's lots of lines but there's also lots of comments and it *is* somewhat stripped down.
```
\RequirePackage[enable-debug]{expl3}

\ExplSyntaxOn
\debug_on:n {check-declarations, check-expressions, deprecation}
\ExplSyntaxOff

\documentclass{article}

\usepackage{pgfcore}
\usepgfmodule{shapes}
\usepackage{nicematrix}

\ExplSyntaxOn

% Average of two dimens.
\cs_new:Nn \nsajko_dim_average:nn {\dim_eval:n {(#1 + #2)/2}}

\msg_new:nnn {just-my-document} {unknown-value} {Unknown~value~given:~#1}

\dim_new:N \g__nsajko_coord_dim

% arg 1:
%   * a PGF point
%
% arg 2:
%   * the chosen dimension, either `x` or `y`
\cs_new_protected:Nn \nsajko_pgf_extract:nn {
  \str_case:nnF
    {#2}
    {
      {x} {\pgfextractx {\g__nsajko_coord_dim} {#1}}
      {y} {\pgfextracty {\g__nsajko_coord_dim} {#1}}
    }
    {\msg_expandable_error:nnn {just-my-document} {unknown-value} {#2~in:~nsajko_pgf_extract:nn}}
  \dim_use:N \g__nsajko_coord_dim
}

% Average of PGF points along either x or y axis.
%
% arg 1:
%   * PGF point
%
% arg 2:
%   * PGF point
%
% arg 3:
%   * the chosen dimension, either `x` or `y`
\cs_new_protected:Nn \nsajko_dim_average_points:nnn {\nsajko_dim_average:nn
  {\nsajko_pgf_extract:nn {#1} {#3}}
  {\nsajko_pgf_extract:nn {#2} {#3}}
}

% arg 1:
%   * x coordinate
%
% arg 2:
%   * y coordinate
%
% arg 3:
%   * the chosen dimension, either `x` or `y`
\cs_new:Nn \nsajko_extract_coord:nnn {
  \str_case:nnF
    {#3}
    {
      {x} {#1}
      {y} {#2}
    }
    {
      \msg_expandable_error:nnn {just-my-document} {unknown-value} {#3~in:~nsajko_extract_coord:nnn}
      -1
    }
}

% Implementation detail, use \nsajko_coord:nnnnn instead.
%
% arg 1:
%   * first node name
%
% arg 2:
%   * first node anchor
%
% arg 3:
%   * second node name
%
% arg 4:
%   * second node anchor
%
% arg 5:
%   * the chosen dimension, either `x` or `y`
\cs_new_protected:Nn \nsajko_coord_impl:nnnnn {\nsajko_dim_average_points:nnn
  % inner
  {\pgfpointanchor {#1} {#2}}

  % outer
  {\pgfpointanchor {#3} {#4}}

  {#5}
}

% Implementation detail, use \nsajko_coord:nnnnn instead.
%
% arg 1:
%   * nicematrix environment name
%
% arg 2:
%   * offset
%
% arg 3:
%   * direction
%
% arg 4:
%   * the chosen dimension, either `x` or `y`
%
% arg 5:
%   * the x coordinate of the matrix element/table cell
%
% arg 6:
%   * the y coordinate of the matrix element/table cell
\cs_new_protected:Nn \nsajko_coord:nnnnnn {\nsajko_coord_impl:nnnnn
  % inner
  {#1 - #5 - #6}
  {#3}

  % outer
  {#1 - \int_eval:n {\nsajko_extract_coord:nnn {#5} {#6} {#4} + #2}}
  {center}

  {#4}
}

\cs_new:Nn \nsajko_coord_helper_lohi_helper_offset:n {
  \str_case:nnF
    {#1}
    {
      {lo} {0}
      {hi} {1}
    }
    {
      \msg_expandable_error:nnn {just-my-document} {unknown-value} {#1~in:~nsajko_coord_helper_lohi_helper_offset:n}
      -1
    }
}

\cs_new:Nn \nsajko_coord_helper_lohi_helper_direction:n {
  \str_case:nnF
    {#1}
    {
      {lo} {north~west}
      {hi} {south~east}
    }
    {
      \msg_expandable_error:nnn {just-my-document} {unknown-value} {#1~in:~nsajko_coord_helper_lohi_helper_direction:n}
      invalid~direction
    }
}

% arg 1:
%   * nicematrix environment name
%
% arg 2:
%   * either `lo` or `hi`, as in "low" and "high"
%
% arg 3:
%   * the chosen dimension, either `x` or `y`
%
% arg 4:
%   * the x coordinate of the matrix element/table cell
%
% arg 5:
%   * the y coordinate of the matrix element/table cell
\cs_new_protected:Nn \nsajko_coord:nnnnn {\nsajko_coord:nnnnnn
  {#1}
  {\nsajko_coord_helper_lohi_helper_offset:n {#2}}
  {\nsajko_coord_helper_lohi_helper_direction:n {#2}}
  {#3}
  {#4}
  {#5}
}

% arg 1:
%   * nicematrix environment name
%
% arg 2:
%   * either `lo` or `hi`, as in "low" and "high", for x
%
% arg 3:
%   * either `lo` or `hi`, as in "low" and "high", for y
%
% arg 4:
%   * the x coordinate of the matrix element/table cell
%
% arg 5:
%   * the y coordinate of the matrix element/table cell
\cs_new_protected:Nn \nsajko_point:nnnnn {\pgfpoint
  % x
  {\nsajko_coord:nnnnn {#1} {#2} {x} {#4} {#5}}

  % y
  {\nsajko_coord:nnnnn {#1} {#3} {y} {#4} {#5}}
}

% arg 1:
%   * nicematrix environment name
%
% arg 2:
%   * the x coordinate of the matrix element/table cell
%
% arg 3:
%   * the y coordinate of the matrix element/table cell
\cs_new_protected:Nn \nsajko_cell_partial_box:nnn {
  \pgfpathmoveto {\nsajko_point:nnnnn {#1} {lo} {hi} {#2} {#3}}

  \pgfpathlineto {\nsajko_point:nnnnn {#1} {hi} {hi} {#2} {#3}}
  \pgfpathlineto {\nsajko_point:nnnnn {#1} {hi} {lo} {#2} {#3}}

  \pgfusepath {stroke}
}

\cs_new_protected:Nn \nsajko_cell_partial_box_drawn:n {
  \pgfrememberpicturepositiononpagetrue

  \begin {pgfpicture}
    \pgfusepath {use~as~bounding~box}
    \nsajko_cell_partial_box:nnn {#1} {1} {1}
  \end {pgfpicture}
}

\NewDocumentCommand {\cellPartialBoxDrawn} {m} {
  \nsajko_cell_partial_box_drawn:n {#1}
}

\ExplSyntaxOff

\begin{document}
  \begin{NiceTabular}{cc}[
    name = synthdivtab,
    columns-width = auto,
    code-after = \cellPartialBoxDrawn{synthdivtab}
  ]
    \( -2 \) & \( -5 \)
  \end{NiceTabular}
\end{document}
```

Why do I get these weird error messages when trying to compile the above:
```
./r.tex:241: Use of \??? doesn't match its definition.
<argument> \???  
      ! Package just-my-document Error: Unknown value given: \def  in: nsajko_p
l.241   \end{NiceTabular}
```

What is the meaning of `\???`?

The end of the error message produced by my code seems to be cut off by the TeX engine for some reason, but nonetheless it seems like the function \\**nsajko_p**gf_extract:nn received `\def` for its second parameter, which should always be either `x` or `y`. `\def` is not a command I ever used directly, but it seems to be the TeX primitive for defining macros, so the thing that commands like `\cs_new:Nn` are probably based on. I don't see how could a function definition find its way into an argument, though.

Any ideas or tips?