(* * Copyright (c) 2009-2010, Andrew Appel, Robert Dockins and Aquinas Hobor. * *)
Definition constructive_join_sub {A} {JOIN: Join A} (w1 w3: A) := {w2 | join w1 w2 w3}.
Hint Resolve @constructive_join_sub_refl : core.
Definition constructive_joins {A}  {JOIN: Join A} (w1 w2 : A) := {w3 | join w1 w2 w3}.

Lemma cjoins_irr {A} `{Perm_alg A}: forall {w1 w2: A}
(j1 j2: constructive_joins w1 w2), proj1_sig j1 = proj1_sig j2.