∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.ExtHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {l : List (α × β)} {k : α},
  (m.insertMany l)[k]? =
    (List.findSomeRev?
          (fun x_2 =>
            match x_2 with
            | (a, b) => if (a == k) = true then some b else none)
          l).or
      m[k]?