proposition: Lemma cardinal_setC3 a b: (a -s b) =c (b -s a) -> a =c b.
proof: Proof. rewrite (cardinal_setC2 (@subsetI2l a b)) (cardinal_setC2 (@subsetI2r a b)). move: (set_CI2 a b a); rewrite setC_v set0_U2 => ->. move: (set_CI2 b a b); rewrite setC_v set0_U2 setI2_C => -> h. by rewrite - csum2cr h csum2cr. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)