Fix Frank's SL2 in nat.rep. code in even char

[fingolfin]

Also add some Asserts for future debugging

[fingolfin]

CC @frankluebeck @Till-Eisen

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.67%. Comparing base (2abdd6c) to head (5608416).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #394   +/-   ##
=======================================
  Coverage   72.67%   72.67%           
=======================================
  Files          43       43           
  Lines       18397    18397           
=======================================
  Hits        13370    13370           
  Misses       5027     5027           

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[frankluebeck]

Thanks for spotting this. The fix looks good to me.