feat: account for mixed predictor specifications

[pat-s]

fixes #54

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.23%. Comparing base (add6f09) to head (a81e415).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   66.09%   66.23%   +0.14%     
==========================================
  Files           5        5              
  Lines         233      234       +1     
==========================================
+ Hits          154      155       +1     
  Misses         79       79              

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