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# Library prosa.util.bigop

From mathcomp Require Export ssreflect ssrnat ssrbool eqtype fintype bigop seq div ssrfun.
Consider a sequence of unique elements xs, an element x0, and a predicate P := λ x ⇒ x == x0. Then, any "bigop" over the sequence xs w.r.t. the predicate and a function F is equal to F x0.
Lemma big_pred1_seq :
∀ [R : Type] [idx : R] [op : Monoid.law idx] [X : eqType] [P : pred X] [F : X → R]
(xs : seq X) (i : X),
(i \in xs) →
(uniq xs) →
P =1 pred1 i →
\big[op/idx]_(j <- xs | P j) F j = F i.
Proof.
move⇒ R idx op X P F xs i IN UNI Pi.
have HAS: has P xs
by apply /hasP; ∃ i ⇒ //=; rewrite (Pi i) pred1E.
move: UNI; have [j s1 s2 Pj nPs1] := (split_find HAS) ⇒ UNI.
have nPs2 : ~~ has P s2.
{ apply: contraT ⇒ /negPn ⇒ hPs2; exfalso.
move: UNI; rewrite cat_uniq //= ⇒ /andP [UNI1 /andP [/negP NE UNI2]].
move: hPs2 ⇒ /hasP [j' IN' Pj'].
apply/NE/hasP; ∃ j' ⇒ //.
move: Pj' Pj; rewrite (Pi j') (Pi j) !pred1E ⇒ /eqP → /eqP →.
by rewrite mem_rcons mem_head. }
rewrite -cats1 big_catl // big_catr // big_cons big_nil.
rewrite ifT // Monoid.simpm.
by move: Pj; rewrite (Pi j) pred1E //= ⇒ /eqP →.
Qed.