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