a bit more context for treerec

[petrelharp]

Here's a few more notes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.43%. Comparing base (0d196d1) to head (8d3f384).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   75.48%   75.43%   -0.06%     
==========================================
  Files         114      114              
  Lines       72405    72405              
  Branches    12804    12804              
==========================================
- Hits        54658    54618      -40     
- Misses      17747    17787      +40     

see 11 files with indirect coverage changes

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