input: Write a function that takes two parameters x and y and returns the sum of these two parameters.
  def add(x, y):
   """Returns the sum of x and y"""
   return x + y
  assert add(2, 3)==5
  assert add(-1, 1)==0
  assert add(0, 0)==0
  assert add(-5, -3)==-8
  assert add(10, -20)==-10
output: def add(x, y):
      return x + y