given a list of token node IDs, returns a their string representation
    (concatenated token strings).

def tokens2text(docgraph, token_ids):
    """
    given a list of token node IDs, returns a their string representation
    (concatenated token strings).
    """
    return ' '.join(docgraph.node[token_id][docgraph.ns+':token']
                    for token_id in token_ids)