reuse the underlying string when charList.Arg(0) is called

[codecov[bot]]

Codecov Report

Base: 98.04% // Head: 97.91% // Decreases project coverage by -0.13% :warning:

Coverage data is based on head (b506d77) compared to base (9a6eb5a). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head b506d77 differs from pull request most recent head ad36045. Consider uploading reports for the commit ad36045 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #250 +/- ## ========================================== - Coverage 98.04% 97.91% -0.14% ========================================== Files 21 21 Lines 7101 7083 -18 ========================================== - Hits 6962 6935 -27 - Misses 109 115 +6 - Partials 30 33 +3 ``` | [Impacted Files](https://codecov.io/gh/ichiban/prolog/pull/250?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase) | Coverage Δ | | |---|---|---| | [engine/compound.go](https://codecov.io/gh/ichiban/prolog/pull/250/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2NvbXBvdW5kLmdv) | `100.00% <100.00%> (ø)` | | | [interpreter.go](https://codecov.io/gh/ichiban/prolog/pull/250/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-aW50ZXJwcmV0ZXIuZ28=) | `94.93% <0.00%> (-4.22%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase)

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