'Turns turtle animation on/off and set delay for update drawings.
Optional arguments:
n -- nonnegative  integer
delay -- nonnegative  integer
If n is given, only each n-th regular screen update is really performed.
(Can be used to accelerate the drawing of complex graphics.)
Second arguments sets delay value (see RawTurtle.delay())
Example (for a TurtleScreen instance named screen):
>>> screen.tracer(8, 25)
>>> dist = 2
>>> for i in range(200):
...     fd(dist)
...     rt(90)
...     dist += 2'

def tracer(self, n=None, delay=None):

    if (n is None):
        return self._tracing
    self._tracing = int(n)
    self._updatecounter = 0
    if (delay is not None):
        self._delayvalue = int(delay)
    if self._tracing:
        self.update()
