fact: Definition comp_hrel A B C (R : A -> B -> Type) (R' : B -> C -> Type) : A -> C -> Type := fun a c => sigT (fun b => R a b * R' b c)%type.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq zmodp) mathcomp(path choice fintype tuple finset bigop)