Arguments cfind {X} A p {p_dec}.
Instance list_forall_dec X A (p : X -> Prop) :
(forall x, dec (p x)) -> dec (forall x, x el A -> p x).
Instance list_exists_dec X A (p : X -> Prop) :
(forall x, dec (p x)) -> dec (exists x, x el A /\ p x).

Lemma list_exists_not_incl (X: eqType) (A B : list X) :
~ A <<= B -> exists x, x el A /\ ~ x el B.