'Adds a turtle shape to TurtleScreen\'s shapelist.
Arguments:
(1) name is the name of a gif-file and shape is None.
Installs the corresponding image shape.
!! Image-shapes DO NOT rotate when turning the turtle,
!! so they do not display the heading of the turtle!
(2) name is an arbitrary string and shape is a tuple
of pairs of coordinates. Installs the corresponding
polygon shape
(3) name is an arbitrary string and shape is a
(compound) Shape object. Installs the corresponding
compound shape.
To use a shape, you have to issue the command shape(shapename).
call: register_shape("turtle.gif")
--or: register_shape("tri", ((0,0), (10,10), (-10,10)))
Example (for a TurtleScreen instance named screen):
>>> screen.register_shape("triangle", ((5,-3),(0,5),(-5,-3)))'

def register_shape(self, name, shape=None):

    if (shape is None):
        if name.lower().endswith('.gif'):
            shape = Shape('image', self._image(name))
        else:
            raise TurtleGraphicsError(('Bad    arguments    for    register_shape.\n' + 'Use        help(register_shape)'))
    elif isinstance(shape, tuple):
        shape = Shape('polygon', shape)
    self._shapes[name] = shape
