input: Write a function that takes two parameters and returns the sum of the two parameters.
  def add(x, y): 
   """Returns the sum of x and y"""
   return x + y
  
  assert add(3, 4) == 7
  assert add(-1, 1) == 0
  assert add(-3, -2) == -5
  assert add(0, 0) == 0
  assert add(10, -10) == 0
output: def add(x, y): 
      return x + y