∀ {α : Type u} {β : α → Type v} (m : Std.DHashMap.Internal.Raw₀ α β) [inst : BEq α] [inst_1 : Hashable α] [EquivBEq α]
  [LawfulHashable α],
  (↑m).WF → ∀ {k : α} {v : β k}, (↑(m.insertIfNew k v)).size = if m.contains k = true then (↑m).size else (↑m).size + 1