input: Write a function to debug a Python code that includes two functions: f1 and f2. The function f1 should increment its input by one and the function f2 should square the result of f1 applied to its input. After debugging, the function f2 should return 9 when applied to the input 2.
  assert f2(2)==9
  assert f2(3)==16
  assert f2(0)==1
  assert f2(-1)==0
  assert f2(10)==121
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) ** 2