'Bind fun to mouse-button-release event on turtle.
fun must be a function with two arguments, the coordinates
of the point on the canvas where mouse button is released.
num, the number of the mouse-button defaults to 1
If a turtle is clicked, first _onclick-event will be performed,
then _onscreensclick-event.'

def _onrelease(self, item, fun, num=1, add=None):

    if (fun is None):
        self.cv.tag_unbind(item, ('<Button%s-ButtonRelease>' % num))
    else:
        def eventfun(event):
            (x, y) = ((self.cv.canvasx(event.x) / self.xscale), ((- self.cv.canvasy(event.y)) / self.yscale))
            fun(x, y)
        self.cv.tag_bind(item, ('<Button%s-ButtonRelease>' % num), eventfun, add)
