Fixes for groupmark validation

[Drvi]

fixes #168

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (41c1bdd) 87.58% compared to head (1b5d26f) 87.58%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   87.58%   87.58%           
=======================================
  Files          10       10           
  Lines        1860     1861    +1     
=======================================
+ Hits         1629     1630    +1     
  Misses        231      231           

Impacted Files	Coverage Δ
src/Parsers.jl	87.29% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[LilithHafner]

Thank you! This looks like it fully and adequately addresses the issue.

[quinnj]

Thanks @Drvi! Especially love all the additional test cases here.