fact: Definition pure_prop3 A S := (forall M N, sub M A -> sub N A -> ~ inc M S -> ~ inc N S -> inc (M \cap N) S -> forall x, inc x (M \cup N) -> ~ (inc ((M \cup N) -s1 x) S)).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)