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
complete the proof of contr_from_Empty
Improve comments in Idempotents
Merge pull request #655 from mikeshulman/contr-from-empty

complete the proof of contr_from_Empty
Merge pull request #651 from mikeshulman/more-idempotents

More idempotents