proposition: Lemma interp_cat i t1 t2 : interp i (t1 ++ t2) = interp i t1 :+ interp i t2.
proof: Proof. elim:t1 t2=>[/=|e t1 IH] t2; first by rewrite un0h. by rewrite cat_cons !interp_cons IH unA. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat ssrfun eqtype seq) LemmaOverloading(prelude prefix perms heaps)