Arguments resource_at_empty [phi] _ _.
Ltac inj_pair_tac :=
match goal with H: (@existT ?U ?P ?p ?x = @existT _ _ _ ?y) |- _ =>
generalize (@inj_pair2 U P p x y H); clear H; intro; try (subst x || subst y)
end.

Lemma age1_ghost_of:
forall phi phi',
age1 phi = Some phi' ->
ghost_of phi' = ghost_fmap (approx (level phi')) (approx (level phi')) (ghost_of phi).