fact: Definition lft : A + B -> option A := fun x => if x is inl x' then Some x' else None.
imports: Coq(ssreflect ssrbool ssrfun Eqdep) mathcomp(ssrnat seq eqtype choice fintype) pcm(options axioms) Coq(Classes.Morphisms Program.Basics Program.Tactics) Coq(Relations)