Code Golf
petStorm
Given two lists only containing positive integers, return all items among these two lists that aren't contained in both of the lists.

## Rules
From the set definition:
> A set is a collection of definite distinct items.

* So you can assume the items in the input lists are always unique.
* You can also take input as a set if your language supports it.

## Test cases
[Here](https://tio.run/##bY2xCsMgFAD39xVv9JFnIG2nUAr6FYI4pEWKEE1RIRTy7zZZSofecNPB@TxP6Sl9eeTwqq1BeUehWBPKG@CONbhtaPAqUTGmpeIcSi1j9PHuszCMmhxg1/2W@n@pyPUAcQpJWPc9hGVcc6heHGs78InPju1uvjiivrUP) is a sample program that generates the test cases.

[1,2,3],[2,3,4] -> [1,4]

ajc2
Lennart Jonsson
EsolangingFruit
Nobody
ajc2
# [Kotlin], 16 bytes

{a,b->a-b+(b-a)}

[Try it online!][TIO-k8z2em2a]

[Kotlin]: https://kotlinlang.org
[TIO-k8z2em2a]: https://tio.run/##y84vycnM@1@WmKNQXJmbkplmpaARnFpi45lXYqejAGNpKujawTkKtv@rE3WSdO0SdZO0NZJ0EzVr/6eV5inkJmbmaWgqVHNxgkxLVLBVKE4t8U/TMNQx0jHWhIgmwUWBYjomQNGCosy8Eg2I3RpAYzU1uWr/AwA "Kotlin – Try It Online"
xigoi
# [Jelly], 2 bytes

œ^

[Try it online!][TIO-khp32yf4]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-khp32yf4]: https://tio.run/##y0rNyan8///o5Lj///9HG@oY6RjH/o8GkjomsQA "Jelly – Try It Online"

Built-in.
Lennart Jonsson
# [Julia 1.0], 24 bytes

julia
symdiff([1,2,3],[2,3,4])


[tio.run](https://tio.run/##yyrNyUw0rPj/v7gyNyUzLU0j2lDHSMc4VicaSOqYxGr@/w8A)
# [APL (Dyalog Unicode)], 3 bytes

apl
∪~∩

[Try it online!][TIO-k9kfg28t]

∪ union ~ without ∩ intersection.

[APL (Dyalog Unicode)]: https://www.dyalog.com/
[TIO-k9kfg28t]: https://tio.run/##SyzI0U2pTMzJT////1HHqrpHHSv/pz1qm/Cot@9RV/Oj3jWPerccWm/8qG3io76pwUHOQDLEwzP4v6GCkYKxQhqYNAEA "APL (Dyalog Unicode) – Try It Online"
EsolangingFruit
# [CJam], 3 bytes

{^}

[Try it online!][TIO-k9hb52gb]

[CJam]: https://sourceforge.net/p/cjam
[TIO-k9hb52gb]: https://tio.run/##S85KzP1fWPe/Oq72f13B/2hDBSMF41iFaCCpYBILAA "CJam – Try It Online"
Nobody
# [J](https://jsoftware.com), 8 bytes

J
,-.[-.-.

[Try it online!](https://tio.run/##y/qvpKdenKJga6WgrqCjYKBgBcS6egrOQT5u/3V09aJ19XT1/mtypSZn5CsYKhgpGCsAVYMoE4SYKUzMHCZmCuRYgESNTRTMQYr/AwA)

Concatenate minus intersection.

Simply concatenates (,) the two arguments and set-subtracts (-.) from it the intersection ($f(A,B)=A{\backslash}(A{\backslash}B)$ ; [ -. -.) of them.

edit: right after posting this, I realized there is a very clever 6-byte solution (I saw it after I checked the answers to this challenge on CGSE). I didn't come up with it myself so I won't count it towards my score, but I'm putting it here anyway.

-.,-.~

Explanation:

Concatenate (,) A\B (-.) and B\A (-. ~)

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