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(2, 3)==5
  assert add(0, 0)==0
  assert add(-1, 1)==0
  assert add(10, -5)==5
  assert add(-3, -3)==-6
output: def add(x, y): 
   return x + y