proposition: Lemma BRsum_0l x: realp x -> \0r +r x = x.
proof: Proof. move => xr. rewrite (BR_sumQ_aux QS0 xr); set_extens t. by move/funI_P =>[z za ->]; rewrite (BQsum_0l (BRi_sQ xr za)). by move => tx; apply/funI_P; ex_tac; rewrite (BQsum_0l (BRi_sQ xr tx)). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)