Adding/Modifying module functions in a previous Livebook cell

I would like to use Livebook to demonstrate building up an increasingly complex module.
Is it possible to add functions without re-specifying the full module in later code cells?

Eg.

defmodule Example do
  def add(a, b) do
    a + b
  end
end
Example.add(1, 2)

3

defmodule Example do
  def sub(a, b) do
    a - b
  end
end
Example.add(1, 2)

** (UndefinedFunctionError) function Example.add/2 is undefined or private
Example.add(1, 2)