fact: Definition BZ_ideal x:= [/\ (forall a, inc a x -> intp a), (forall a b, inc a x -> inc b x -> inc (a +z b) x) & (forall a b, inc a x -> intp b -> inc (a *z b) x)].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)