input: Write a function to debug the provided Python code. Ensure that the function `f1` returns a value and that `f2` computes the square of `f1(x)`. The function should be tested with the input `f2(2)`, and the output should be `9`.
  assert f2(2) == 9
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) ** 2