input: Write a function to return the sum of two parameters 'x' and 'y'.
  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, -7) == 3
output: def add(x, y):
      return x + y