fact: Definition quasi_bij (f: fterm) I J := [/\ (forall x, inc x I -> inc (f x) J), (forall x y, inc x I -> inc y I -> f x = f y -> x = y) & (forall y, inc y J -> exists2 x, inc x I & y = f x)].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)