  def add(x,y,z):
    return x+y+z
  def subtract(x,y):
    pass
  def multiply(x,y,z):
    return x*y*z
  def divide(x,y):
   return x/y
  def square(x,y):
    pass
  def multiply(x,y,z,a):
   return x*y*z*a

