input: Write a python function to create a factorial program string.
  assert create_factorial_program() == '''def factorial(n):
      if n < 0:
          return None
      elif n == 0:
          return 1
      else:
          return n * factorial(n - 1)'''
output: def create_factorial_program():
      return '''def factorial(n):
      if n < 0:
          return None
      elif n == 0:
          return 1
      else:
          return n * factorial(n - 1)'''