Opaque member_dec.

Lemma union_pred_derives_const: forall m {A} (P: forall it, A it -> val -> mpred) p v R,
members_no_replicate m = true ->
m <> nil ->
(forall i (v: A (i, field_type i m)), in_members i m -> P _ v p |-- R) ->
union_pred m P v p |-- R.