input: Write a function to debug the given Python code. The function `f1` should return the value of `x + 1`. The function `f2` should return the square of the value returned by `f1` when applied to `x`. Finally, call `f2(2)` and print the output.
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  print(f2(2))
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) ** 2