input: Write a function to debug a given Python code that defines two functions f1 and f2. The function f1 should return the input x plus 1, and the function f2 should return the square of the result of calling f1 with the input x. The function should ensure that f1 correctly returns a value and f2 correctly uses f1 to compute the result. The assert statements should check the correctness of the code.
  def debug_functions():
      def f1(x):
          return x + 1
  
      def f2(x):
          return f1(x) * f1(x)
  
      assert f2(2) == 9
      assert f2(-1) == 0
      assert f2(0) == 1
      assert f2(-5) == 4
      assert f2(10) == 121
output: def debug_functions():
      def f1(x):
          return x + 1
  
      def f2(x):
          return f1(x) ** 2
  
      assert f2(2) == 9
      assert f2(-1) == 0
      assert f2(0) == 1
      assert f2(-5) == 4
      assert f2(10) == 121