proposition: Lemma subctx_trans j i k : subctx i j -> subctx j k -> subctx i k.
proof: Proof. move=>[H1 P1][H2 P2]. by split; [move: H2|move: P2]; apply: prefix_trans. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat ssrfun eqtype seq) LemmaOverloading(prelude prefix perms heaps)