input: Write a function to debug the given Python code for the functions f1 and f2, and ensure that f1 correctly returns the increment of the input and that f2 returns the square of the increment. The function should print the result when f2 is called with the argument 2.
  
  # define f1 #
  def f1(x):
      return x + 1
  
  # define f2 #
  def f2(x):
      return f1(x) * f1(x)
  
  # call f2 #
  print(f2(2)) #Output: 9
  
  assert f1(3) == 4
  assert f2(2) == 9
  assert f1(-1) == 0
  assert f2(-1) == 0
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)