Definition opt_t_aux (x: option (elt * tree))
(P: match x with Some (_,s) => valid s | None => True end) : option (elt * t) :=
match
x as o
return
(match o with
| Some (_, s) => valid s
| None => True
end -> option (elt * t))
with
| Some (k,s') => fun P0 : valid s' => Some (k, exist valid s' P0)
| None => fun _ : True => None
end P.
Definition delete_min (s: t) : option (elt * t) :=
opt_t_aux (delete'_min (proj1_sig s)) (delete_min_valid _ (proj2_sig s)).

Lemma delete_min_spec1: forall (s: t) k s',
delete_min s = Some (k,s') <->
(min_elt s = Some k /\ remove k s = s').