Opaque member_dec.

Lemma struct_pred_not_member: forall m {A} (P: forall it, A it -> val -> mpred) (i: ident) v p,
let P' it := if ident_eq i (fst it) then fun _ _ => emp else P it in
~ in_members i m ->
struct_pred m P v p = struct_pred m P' v p.