input: Write a function to ensure the Python function `f2(x)` correctly multiplies the result of `f1(x)` by itself and returns the correct value. Make sure `f1(x)` is defined to return the increment of `x`.
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  print(f2(2))  # Output: 9
output: def f2(x):
      return f1(x) * f1(x)