or
string array-manipulation source-rules
Given a list or string (you can decide to handle only one of these), return it unmodified. However, if your source code is palindromised by appending its reverse to it, then the this new code must palindromise the given list or string.

For the purposes of this competition, you must count a trailing line break if it is necessary to make your code run when palindromised.

## Examples
Let's say your solution takes a list and your source code is

AB
DEF

and its input/argument is [1,2,3]. If I write

AB
DEFFED
BA

instead and run it, the output/result must be [1,2,3,3,2,1].

Let's say your solution takes a string and your source code is

ABC


and its input/argument is "42\n". If I write

ABC

CBA

instead and run it, the output/result must be "42\n\n24".
liek
# [W](https://github.com/A-ee/w/) t, 4 bytes

+_:a

# Explanation

+    % Join 2 (implicit) inputs
_   % Reverse the joined input
:  % Copy the input
a % All of this is discarded after the input push

# Explanation-noitanalpxE

+_:a % Stack is as above

a    % Push the input
:   % Copy the input
_  % Reverse the copy
+ % Join to the original string
% I.e. palindromizing the input

Bubbler
# [APL (Dyalog Unicode)], 3 bytes^SBCS^

⌽⊢,

[Try it online!][TIO-k89pozol]

[APL (Dyalog Unicode)]: https://www.dyalog.com/
[TIO-k89pozol]: https://tio.run/##SyzI0U2pTMzJT////1HP3kddi3T@pz1qm/Cot@9R31RP/0ddzYfWGz9qmwjkBQc5A8kQD8/g/2kK6olJyepcQNrA0MjYxNTM3MJSHQA "APL (Dyalog Unicode) – Try It Online"

## Palindromized

⌽⊢,,⊢⌽

[Try it online!!enilno ti yrT][TIO-k89ppz0r]

[APL (Dyalog Unicode)]: https://www.dyalog.com/
[TIO-k89ppz0r]: https://tio.run/##SyzI0U2pTMzJT////1HP3kddi3R0gASQ@T/tUduER719j/qmevo/6mo@tN74UdtEIC84yBlIhnh4Bv9PU1BPTEpW5wLSBoZGxiamZuYWluoA "APL (Dyalog Unicode) – Try It Online"

### How it works

⌽⊢,  ⍝ Original function; Input = s
⌽ ,  ⍝ Compute reverse(s) and flatten(s) (flatten is a no-op)
⊢   ⍝ Select the right one, i.e. flatten(s)

⌽⊢,,⊢⌽  ⍝ Palindromized function; Input = s
,⊢⌽  ⍝ Similar to the original, but select reverse(s)
⊢,     ⍝ Append the above to s, giving s+reverse(s)
⌽       ⍝ Reverse the entire string, which is no-op on a palindrome
liek
## [05AB1E], 3 bytes

«ÂI

[Try it online!][TIO-k78bvage]

[TIO-k78bvage]: https://tio.run/##yy9OTMpM/f//0OrDTZ7//0cb6hjpGMcCAA "05AB1E – Try It Online"
## Explanation

«   Concatenate the (implicit) inputs
Â  Bifuricate: Push a, reversed(a)
I Push the input
Implicitly output the top of the stack

## Explanation-noitanalpxE

«ÂI Same as above

I   Push the input
Â  Bifuricate: Push a, reversed(a)
« Concatenate them
Implicitly output the top of the stack

I palindromise the source code, you palindromise the input!
liek
Sorry, I have mistakened the test cases. The test case has no problems... (Will be trying to delete all of these chat messages.)
liek
trailing line break? What does that mean?
liek
(By the way my program works for both ways of palindromizing.)
liek
Can you clarify what exactly does your palindromizing the source means? It seems to be just a copy+reverse when it's applied to the first test case, however there's only 1 newline under the line ABC instead of two under the second example.
Well spotted. I guess our threshold for opening challenges to answers is too low. Neither voter spotted the obvious mistake in the example outputs.
Are you sure that the input is just reversed after palindromizing the code? Because I realized that completing the challenge is quite trivial with it.