input: Write a function to debug the given Python code. The function should define a function f1 that takes an integer x and returns x + 1, and a function f2 that takes an integer x and returns the square of the result of f1(x). The function should then call f2 with an argument of 2 and print the result.
  
  ### Function definition:
  def debug_code():
      # define f1
      def f1(x):
          return x + 1
      
      # define f2
      def f2(x):
          return f1(x) * f1(x)
      
      # call f2
      print(f2(2))
  
  ### Assert tests:
  assert debug_code() == None
output: def debug_code():
      def f1(x):
          return x + 1
      
      def f2(x):
          return f1(x) * f1(x)
      
      print(f2(2))