'Bind fun to mouse-move event on this turtle on canvas.
Arguments:
fun -- a function with two arguments, to which will be assigned
the coordinates of the clicked point on the canvas.
num -- number of the mouse-button defaults to 1 (left mouse button).
Every sequence of mouse-move-events on a turtle is preceded by a
mouse-click event on that turtle.
Example (for a Turtle instance named turtle):
>>> turtle.ondrag(turtle.goto)
Subsequently clicking and dragging a Turtle will move it
across the screen thereby producing handdrawings (if pen is
down).'

def ondrag(self, fun, btn=1, add=None):

    self.screen._ondrag(self.turtle._item, fun, btn, add)
