change internal representations for atoms

[codecov[bot]]

Codecov Report

Base: 98.21% // Head: 98.22% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (5eb3d5d) compared to base (04c5dcf). Patch coverage: 99.04% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #259 +/- ## ======================================= Coverage 98.21% 98.22% ======================================= Files 22 22 Lines 7174 7199 +25 ======================================= + Hits 7046 7071 +25 Misses 102 102 Partials 26 26 ``` | [Impacted Files](https://codecov.io/gh/ichiban/prolog/pull/259?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase) | Coverage Δ | | |---|---|---| | [engine/env.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2Vudi5nbw==) | `98.90% <85.71%> (ø)` | | | [engine/dcg.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2RjZy5nbw==) | `96.10% <96.66%> (ø)` | | | [engine/builtin.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2J1aWx0aW4uZ28=) | `97.72% <98.67%> (+<0.01%)` | :arrow_up: | | [cmd/1pl/interpreter.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-Y21kLzFwbC9pbnRlcnByZXRlci5nbw==) | `100.00% <100.00%> (ø)` | | | [engine/atom.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2F0b20uZ28=) | `100.00% <100.00%> (ø)` | | | [engine/clause.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2NsYXVzZS5nbw==) | `98.61% <100.00%> (ø)` | | | [engine/compound.go](https://codecov.io/gh/ichiban/prolog/pull/259/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)` | | | [engine/exception.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2V4Y2VwdGlvbi5nbw==) | `100.00% <100.00%> (ø)` | | | [engine/iterator.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2l0ZXJhdG9yLmdv) | `100.00% <100.00%> (ø)` | | | [engine/number.go](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL251bWJlci5nbw==) | `100.00% <100.00%> (ø)` | | | ... and [8 more](https://codecov.io/gh/ichiban/prolog/pull/259/diff?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase) | | 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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