proposition: Lemma Eq_sumC A B: dsum A B \Eq dsum B A.
proof: Proof. move:C0_ne_C1 C1_ne_C0 => ha hb; rewrite ! candu2_rw setU2_C. apply: Eq_du2; fprops ;apply: Eq_ii1. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)