∀ {β : Type u_1} {α : Type u_2} [inst : LE β] [inst_1 : DecidableLE β] (f : α → β) {xs ys : List α} {i j : ℕ}
  (hi : i < xs.length) (hj : j < ys.length), List.combineMinIdxOn✝ f i j hi hj < (xs ++ ys).length