add tests and fix bugs in modrem

[araujoms]

As it turns out, the untested code was buggy.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.73%. Comparing base (674be88) to head (af3618f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
+ Coverage   65.80%   67.73%   +1.92%     
==========================================
  Files          54       54              
  Lines        2445     2442       -3     
==========================================
+ Hits         1609     1654      +45     
+ Misses        836      788      -48     

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