fact: Definition funsetU U X Y Z := U Z /\ (forall f, inc f Z <-> [/\ U f, (forall p, inc p f -> inc p (Y \times X)), (forall x, inc x Y -> exists2 y, U y & inc (J x y) f) & (forall x y y', inc (J x y) f -> inc (J x y') f -> y = y') ]).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)