input: Write a function to generate a Python program that adds two numbers and then doubles the result.
  assert double_add(1, 2)==6
  assert double_add(3, 4)==14
  assert double_add(0, 0)==0
  assert double_add(-1, 1)==0
  assert double_add(-2, -2)==-8
  assert double_add(5, 5)==20
output: def double_add(a, b):
      return (a + b) * 2