proposition: Lemma big_ord0 : forall P F, \big[op/idx]_(i < 0 | P i) F i === idx.
proof: Proof. by move=> P F; rewrite -> big_pred0 => [|[]]; try reflexivity. Qed.
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq div choice fintype finfun paths Setoid Morphisms OperationClasses bigops