docs: def tokenize(self, tw):
          """ Given a Tweet object, return a dict mapping field name to tokens. """
          toks = defaultdict(lambda: [])
          for field in self.fields:
              if '.' in field:
                  parts = field.split('.')
                  value = tw.js
                  for p in parts:
                      value = value[p]
              else:
                  value = tw.js[field]
              toks[field].extend(self.do_tokenize(value))
          return dict([(f, v) for f, v in toks.items()])