code: function(fill) {
      var xml = this.map.fill.toXml(fill);
      var index = this.index.fill[xml];
      if (index === undefined) {
        index = this.index.fill[xml] = this.model.fills.length;
        this.model.fills.push(xml);
      }
      return index;
    }
docs: ========================================================================= Fills