proposition: Lemma big_ord_narrow_leq n1 n2 F (le_n12 : n1 <= n2) : let w := @widen_ord n1.+1 n2.+1 le_n12 in \big[op/idx]_(i < n2.+1 | i <= n1) F i = \big[op/idx]_(i < n1.+1) F (w i).
proof: Proof. exact: (big_ord_narrow_cond_leq (predT)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)