input: Write a function to debug the following Python code and ensure that the function f2 correctly returns the square of the increment of its input. Define both functions f1 and f2 properly, and print the output of f2 when called with the input 2. 
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  assert f2(2) == 9
  assert f2(3) == 16
  assert f2(0) == 1
  assert f2(-1) == 0
  assert f2(-2) == 1
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) ** 2