fact: Definition f_set (E: gset A) : gset B := fun y => exists2 x, E x & eqB (f x) y.
imports: Coq(RelationClasses Setoid Morphisms SetoidList Mergesort Permutation) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq fintype) mathcomp(bigop path) RegexpBrzozowski(glue)