add bymap₊

[aplavin]

Following up on #162. I already found myself copy-pasting this function several times, would be nice to have it here in MCM.

Better naming suggestions welcome :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (0455a9c) to head (040eed0).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
+ Coverage   89.46%   89.80%   +0.33%     
==========================================
  Files          20       21       +1     
  Lines        1168     1187      +19     
==========================================
+ Hits         1045     1066      +21     
+ Misses        123      121       -2     

Flag	Coverage Δ
unittests	89.80% <100.00%> (+0.33%)	⬆️

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[baggepinnen]

Thanks for the PR :)
Any name without the awkward ₊ character would be good, bymap2, bymap_a etc.

[aplavin]

renamed!

[aplavin]

any more thoughts on how best to proceed here?
I'd be quite happy to stop copy-pasting this every once in a while :)

[baggepinnen]

This functionality is sufficiently complex that I cannot look at the implementation and determine when it works and when it doesn't. It would be nice to have a few more test cases to make sure it doesn't produce the wrong result for complex data structures. There are a few test cases here

• more general bymap (RFC) #162 (comment)
• more general bymap (RFC) #162 (comment)

it's okay if it doesn't handle all cases, but we should at least ensure that it errors rather than returning the wrong result

[aplavin]

Makes sense! Added more tests.