def _parse_sentencetree(self, tree, parent_node_id=None, ignore_traces=True):
        """parse a sentence Tree into this document graph"""
        def get_nodelabel(node):
            if isinstance(node, nltk.tree.Tree):
                return node.label()
            elif isinstance(node, unicode):
                return node.encode('utf-8')
            else:
                raise ValueError("Unexpected node type: {0}, {1}".format(type(node), node))

        root_node_id = self._node_id
        self.node[root_node_id]['label'] = get_nodelabel(tree)

        for subtree in tree:
            self._node_id += 1
            node_label = get_nodelabel(subtree)
            # unescape the node label, if necessary
            node_label = PTB_BRACKET_UNESCAPE.get(node_label, node_label)
            # TODO: refactor this, so we don't need to query this all the time
            if ignore_traces and node_label == '-NONE-': # ignore tokens annotated for traces
                continue
            if isinstance(subtree, nltk.tree.Tree):
                if len(subtree) > 1: # subtree is a syntactic category
                    node_attrs = {'label': node_label,
                                  self.ns+':cat': node_label}
                    layers = {self.ns, self.ns+':syntax'}
                else:  # subtree represents a token and its POS tag
                    node_attrs = {'label': node_label}
                    layers = {self.ns}

                edge_type = dg.EdgeTypes.dominance_relation
                self.add_node(self._node_id, layers=layers,
                              attr_dict=node_attrs)
                self.add_edge(root_node_id, self._node_id, edge_type=edge_type)

            else: # isinstance(subtree, unicode); subtree is a token
                # we'll have to modify the parent node of a token, since
                # in NLTK Trees, even a leaf node (with its POS tag) is
                # represented as a Tree (an iterator over a single unicode
                # string), e.g. ``Tree('NNS', ['prices'])``
                pos_tag = self.node[parent_node_id]['label']
                token_attrs = {
                    'label': node_label, self.ns+':token': node_label,
                    self.ns+':pos': pos_tag}
                self.node[parent_node_id].update(token_attrs)
                self.tokens.append(parent_node_id)

            if isinstance(subtree, nltk.tree.Tree):
                self._parse_sentencetree(subtree, parent_node_id=self._node_id)
parse a sentence Tree into this document graph