Canonical Structure union_form s1 s2 h1 h2 :=
Form (@union_pf s1 s2 h1 h2).

Lemma foundz_pf x y z (s : form x y) : axiom x y (foundz_tag (z :: s)).
Canonical Structure foundz_form x y z (s : form x y) :=
Form (foundz_pf z s).