input: Write a function to debug the provided Python code by fixing the issues and ensuring the functions f1 and f2 work correctly. The function f1 should return the increment of x by 1, and the function f2 should return the square of f1 applied to x.
  assert f1(1)==2
  assert f2(2)==9
  assert f1(5)==6
  assert f2(-1)==0
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) ** 2