code: def spanstring2text(docgraph, span_string):
      
      token_node_ids = spanstring2tokens(docgraph, span_string)
      return u' '.join(docgraph.node[tok_node_id][docgraph.ns+':token']
                       for tok_node_id in token_node_ids)
docs: converts a span of tokens (str, e.g. 'word_88..word_91') into a string
      that contains the tokens itself.