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