input: Write a function to debug the given Python code where the function `f1` needs to return a value and the function `f2` needs to call `f1` and return the square of the value returned by `f1`. If the corrected code is passed the argument `2`, it should return `9`.
  
  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) ** 2