code: function tokensId(tokens, prepend) {
      if (!prepend) return null;
  
      return _.reduce(tokens, function(prev, token) {
          var tokId = [
              token.index,
              token.offset,
              token.value
          ].join('-');
  
          return [prev, tokId].join(':');
      }, prepend);
  }
docs: /*
  Token is an object like:
  
  {
  "value": "text content",
  
  Position of the index
  "index": 0,
  
  Length of the token
  "offset": "text content".length
  }
   Return a string ID to represent a list of tokens