proposition: Lemma lowUn h1 h2 t1 t2 : def (h1 :+ t1) -> def (h2 :+ t2) -> h1 =~ h2 -> t1 =~ t2 -> h1 :+ t1 =~ h2 :+ t2.
proof: Proof. move=>D1 D2; do 2![case: loweqP=>//]=>H1 H2 _ _. apply/loweqP=>x; move: (H1 x) (H2 x). by rewrite !ldomP !domUn !inE D1 D2 /= !andb_orl=>-> ->. Qed.
imports: mathcomp(ssreflect ssrfun ssrnat div ssrbool seq) LemmaOverloading(prelude finmap ordtype) mathcomp(path eqtype) Eqdep