input: Write a function to debug the given Python code by defining the missing return statement in function `f1` and ensuring the function `f2` correctly uses `f1`. Then call `f2` with the argument `2` and print the result.
  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) * f1(x)