code: function(style) {
      var xml = this.map.style.toXml(style);
      var index = this.index.style[xml];
      if (index === undefined) {
        index = this.index.style[xml] = this.model.styles.length;
        this.model.styles.push(xml);
      }
      return index;
    }
docs: ========================================================================= Private Interface