Opaque member_dec.

Lemma struct_pred_upd: forall m {A} (P: forall it, A it -> val -> mpred) (i: ident) v p v0,
let P' it := if ident_eq i (fst it) then fun _ _ => emp else P it in
members_no_replicate m = true ->
in_members i m ->
struct_pred m P (upd_struct i m v v0) p = P _ v0 p * struct_pred m P' v p.