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