Definition only_blocks {S: block -> Prop} (S_dec: forall b, {S b}+{~S b}) (w: rmap) : rmap.Definition not_dec: forall {S: block -> Prop} (f: forall b, {S b}+{~S b}),
                            forall b, {~S b}+{~ ~ S b}.

Lemma Exists_dec: forall {T} (f: T -> Prop)(f_dec: forall x, {f x}+{~f x}) (l: list T), {Exists f l}+{~Exists f l}.