∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.ExtHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] [inst_2 : Inhabited α] {k : α} {f : β → β},
  (m.modify k f).getKey! k = if k ∈ m then k else default