∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.DHashMap α β} [inst : LawfulBEq α] {k : α}
  {fallback : β k} {f : β k → β k}, (m.modify k f).getD k fallback = (Option.map f (m.get? k)).getD fallback