∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m : Std.ExtDHashMap α β} [inst : LawfulBEq α]
  [inst_1 : Inhabited α] {k : α} {f : β k → β k}, (m.modify k f).getKey! k = if k ∈ m then k else default