splitting already-split idempotents
an endomap splits through at most one type
recovering the witness of idempotency from the splitting
the type of retractions is a retract of partially-coherent idempotents
big reworking of Idempotents
cbn works where simpl fails
Improve comments in Idempotents
Merge pull request #651 from mikeshulman/more-idempotents

More idempotents