fix truncation of already visited compounds in writeCommpoundVisit().

[ichiban]

https://github.com/ichiban/prolog/issues/297

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (5ad402c) 99.18% compared to head (07b054c) 99.18%.

:exclamation: Current head 07b054c differs from pull request most recent head c6a441e. Consider uploading reports for the commit c6a441e to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #298 +/- ## ========================================== - Coverage 99.18% 99.18% -0.01% ========================================== Files 23 23 Lines 7221 7217 -4 ========================================== - Hits 7162 7158 -4 Misses 53 53 Partials 6 6 ``` | [Impacted Files](https://app.codecov.io/gh/ichiban/prolog/pull/298?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://app.codecov.io/gh/ichiban/prolog/pull/298?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2NvbXBvdW5kLmdv) | `94.31% <100.00%> (-0.08%)` | :arrow_down: | | [engine/term.go](https://app.codecov.io/gh/ichiban/prolog/pull/298?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)` | |

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