Definition midObj {A} {JA: Join A} (a : A) : Prop := ~identity a /\ ~ full a.Definition ijoinable A {JA: Join A} : Type := {sh : A & midObj sh}.Definition ijoin {A} {JA: Join A} (j1 j2 j3 : ijoinable A) : Prop :=
  match (j1, j2, j3) with
  (existT _ t1 _, existT _ t2 _, existT _ t3 _) => join t1 t2 t3
  end.

Lemma ijoin_canc {A} {JA: Join A}{SA: Sep_alg A}{CA: Canc_alg A}: forall a a' b c, ijoin a b c -> ijoin a' b c -> a = a'.