input: Write a function to debug the given Python code, ensuring that the function definitions are correct and the final output is as expected. The function f1 should return the input incremented by 1, while function f2 should call f1 twice and return the square of the incremented value.
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  print(f2(2)) #Output: 9
  
  assert f1(2) == 3
  assert f2(2) == 9
  assert f1(0) == 1
  assert f2(0) == 1
  assert f1(-1) == 0
  assert f2(-1) == 1
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)