'Print trace output displaying the parser\'s current state.
:param operation: A character identifying the operation that
generated the current state.
:rtype: None'

def _trace_tree(self, tree, frontier, operation):

    if (self._trace == 2):
        print((u'        %c    [' % operation), end=u'    ')
    else:
        print(u'                [', end=u'    ')
    if (len(frontier) > 0):
        self._trace_fringe(tree, frontier[0])
    else:
        self._trace_fringe(tree)
    print(u']')
