input: Write a function to add two numbers together and print the result.
  def add(a, b):
      sum = a + b
      print(sum)
  
  assert add(5, 7)
  assert add(3, 4)
  assert add(0, 0)
  assert add(-1, 1)
  assert add(-5, -7)
output: def add(a, b):
      sum = a + b
      print(sum)