fact: Definition setoid_cancel {A : Type} {B : Type} `{Equivalence A aeq} (f : A -> B) g := forall x, g (f x) === x.
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq choice fintype finfun bigops ssralg groups perm zmodp morphisms Ring RingClass bigopsClass Setoid Morphisms