Add comments to sexp.rs

[matt-o-how]

Also update a couple of variable names

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 5799703140

• 59 of 73 (80.82%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 78.446%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/compiler/sexp.rs	59	73	80.82%
<!--	Total:	59	73	80.82%	-->

Files with Coverage Reduction	New Missed Lines	%
src/compiler/sexp.rs	9	78.05%
<!--	Total:	9		-->

Totals
Change from base Build 5798509988:	-0.07%
Covered Lines:	7308
Relevant Lines:	9316

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💛 - Coveralls

[prozacchiwawa]

give cargo fmt a run, then will re-approve.