'A generator that implements the actual parsing algorithm.
``step`` iterates through this generator, and restarts it
whenever the parser\'s strategy, grammar, or chart is modified.'

def _parse(self):

    chart = self._chart
    grammar = self._grammar
    edges_added = 1
    while (edges_added > 0):
        edges_added = 0
        for rule in self._strategy:
            self._current_chartrule = rule
            for e in rule.apply_everywhere(chart, grammar):
                edges_added += 1
                (yield e)
