input: Write a function to sum two numbers provided as parameters and return the result.
  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(0, 0) == 0
  assert add(5, -3) == 2
  assert add(-5, -5) == -10
output: def add(x, y):
      return x + y