proposition: Lemma bigIn (xs : seq I) a i v : valid (\big[PCM.join/Unit]_(i <- xs) f i) -> i \In xs -> (a, v) \In f i -> (a, v) \In \big[PCM.join/Unit]_(i <- xs) f i.
proof: Proof. by move=>V H /In_find/(big_find_some V H)/In_find. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)