∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.DHashMap α β} [EquivBEq α] [LawfulHashable α]
  {l : List ((a : α) × β a)},
  List.Pairwise (fun a b => (a.fst == b.fst) = false) l →
    (∀ a ∈ m, (List.map Sigma.fst l).contains a = false) → (m.insertMany l).size = m.size + l.length