'Start parsing a given text.  This sets the parser\'s tree to
the start symbol, its frontier to the root node, and its
remaining text to ``token[\'SUBTOKENS\']``.'

def initialize(self, tokens):

    self._rtext = tokens
    start = self._grammar.start().symbol()
    self._tree = Tree(start, [])
    self._frontier = [()]
    self._tried_e = {}
    self._tried_m = {}
    self._history = []
    self._parses = []
    if self._trace:
        self._trace_start(self._tree, self._frontier, self._rtext)
