proposition: Lemma span_bigcat : (<<\big[cat/[::]]_(i | P i) Xs i>> = \sum_(i | P i) <<Xs i>>)%VS.
proof: Proof. by rewrite (big_morph _ span_cat span_nil). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop finfun tuple) mathcomp(ssralg matrix mxalgebra zmodp)