change internal representations for variables

[codecov[bot]]

Codecov Report

Base: 98.20% // Head: 98.21% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (da30f61) compared to base (853ed28). Patch coverage: 98.43% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #258 +/- ## ======================================= Coverage 98.20% 98.21% ======================================= Files 22 22 Lines 7140 7174 +34 ======================================= + Hits 7012 7046 +34 Misses 102 102 Partials 26 26 ``` | [Impacted Files](https://codecov.io/gh/ichiban/prolog/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase) | Coverage Δ | | |---|---|---| | [cmd/1pl/interpreter.go](https://codecov.io/gh/ichiban/prolog/pull/258/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% <ø> (ø)` | | | [engine/clause.go](https://codecov.io/gh/ichiban/prolog/pull/258/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% <ø> (-0.02%)` | :arrow_down: | | [engine/variable.go](https://codecov.io/gh/ichiban/prolog/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3ZhcmlhYmxlLmdv) | `91.17% <96.66%> (+4.81%)` | :arrow_up: | | [engine/builtin.go](https://codecov.io/gh/ichiban/prolog/pull/258/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% <100.00%> (ø)` | | | [engine/env.go](https://codecov.io/gh/ichiban/prolog/pull/258/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% <100.00%> (+0.02%)` | :arrow_up: | | [engine/parser.go](https://codecov.io/gh/ichiban/prolog/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3BhcnNlci5nbw==) | `98.65% <100.00%> (ø)` | | | [engine/term.go](https://codecov.io/gh/ichiban/prolog/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3Rlcm0uZ28=) | `100.00% <100.00%> (ø)` | | | [engine/vm.go](https://codecov.io/gh/ichiban/prolog/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3ZtLmdv) | `86.95% <100.00%> (-0.04%)` | :arrow_down: | | [solutions.go](https://codecov.io/gh/ichiban/prolog/pull/258/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-c29sdXRpb25zLmdv) | `100.00% <100.00%> (ø)` | | 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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