proposition: Lemma big_ord_narrow n1 n2 F (le_n12 : n1 <= n2) : let w := widen_ord le_n12 in \big[op/idx]_(i < n2 | i < n1) F i = \big[op/idx]_(i < n1) F (w i).
proof: Proof. exact: (big_ord_narrow_cond (predT)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)