fact: Definition filterf f (P : pred K) : {fmap K -> V} := [fmap x : [fset x in domf f | P x] => f (fincl (fset_sub _ _) x)].
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)