input: Write a function to debug the given Python code where `f1` does not return a value, and `f2` calls `f1` to calculate the square of `f1(x)`. The function should ensure that `f1` returns the correct value and that `f2` correctly computes the square of `f1(x)`.
  
  def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)
  
  assert f2(2)==9
  assert f2(0)==1
  assert f2(-1)==0
  assert f2(-2)==1
  assert f2(1)==4
output: def f1(x):
      return x + 1
  
  def f2(x):
      return f1(x) * f1(x)