∀ {α : Type u} {β : Type v} [inst : BEq α] [inst_1 : Hashable α] {m : Std.HashMap.Raw α β} [inst_2 : EquivBEq α]
  [LawfulHashable α] {k : α} [inst_4 : Inhabited β] {f : β → β}, m.WF → (m.modify k f)[k]! = (Option.map f m[k]?).get!