def _reset(self):
self._segments = <NUM_LIT:0><EOL>self._duration = <NUM_LIT:0><EOL>
Resets the number of drawn segments and the duration.

        To calculate the number of segments or the total time needed,
        we need to recurse through LSystem._grow().
        Before that, the draw(), segments() and duration() command
        will reset both tally variables.