Add `!` suffix to functions that modify their arguments

[sloede]

Fixes #3

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f79915e) 86.09% compared to head (e99f573) 86.09%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6 +/- ## ======================================= Coverage 86.09% 86.09% ======================================= Files 7 7 Lines 187 187 ======================================= Hits 161 161 Misses 26 26 ```

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