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