docs: function(sass) {
      return sass.replace(/\$([\w-]+)\s*([^:;\s]*;)/g, function(match, fieldName, terminus) {
        if (this.vars.hasOwnProperty(fieldName)) {
          this._addFieldUsage(fieldName);
          return this.fieldOpen + fieldName + this.fieldClose + terminus;
        }
        return match;
      }.bind(this));
    }