input: def double_add(num1, num2):
      """
      Add two numbers and then double the result.
      
      >>> double_add(1, 2)
      6
      >>> double_add(0, 0)
      0
      >>> double_add(-1, -1)
      -4
      """
      return (num1 + num2) * 2
output:     return (num1 + num2) * 2