def __isub__ (self, other):
    """Subtracts the (x, y) coordinates of two points or a point and a
    number.  Examples:

    >>> p  = Point(1, 2)
    >>> p -= 1
    >>> p
    Point(0, 1)

    >>> p  = Point(1, 2)
    >>> p -= Point(3, 4)
    >>> p
    Point(-2, -2)
    """
    if isinstance(other, Point):
      self.x -= other.x
      self.y -= other.y
      if self.z and other.z:
        self.z -= other.z
    else:
      self.x -= other
      self.y -= other
      if self.z:
        self.z -= other
    return self
def __isub__ ( self , other ) : if isinstance ( other , Point ) : self . x -= other . x self . y -= other . y if self . z and other . z : self . z -= other . z else : self . x -= other self . y -= other if self . z : self . z -= other return self
Subtracts the (x, y) coordinates of two points or a point and a
    number.  Examples:

    >>> p  = Point(1, 2)
    >>> p -= 1
    >>> p
    Point(0, 1)

    >>> p  = Point(1, 2)
    >>> p -= Point(3, 4)
    >>> p
    Point(-2, -2)