input: Write a function to debug the given Python code that defines two functions `f1` and `f2`. The `f1` function should return the result of `x + 1` and the `f2` function should return the product of two invocations of `f1` with the same argument. Ensure that the function `f1` is properly defined to return a value.
  
  assert f2(2) == 9
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)