Use bool flag in open_operators!

[lukem12345]

This is a tiny PR that uses a bool in open_operators! instead of an index, just to be explicit. No behavior should change.

This might be a rare opportunity to use a fall-through switch statement, since all but one case returns true. But of course that's not completely necessary.

We have discovered that some open_operators tests were lost during the DiagrammaticEquations split, and those are now recovered in this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.09%. Comparing base (10695ac) to head (d77ec31).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #249       +/-   ##
===========================================
+ Coverage   67.69%   78.09%   +10.40%     
===========================================
  Files           5        5               
  Lines         616      630       +14     
===========================================
+ Hits          417      492       +75     
+ Misses        199      138       -61     

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