docs: function (expression, descending, opts) {
              descending = isBoolean(descending) ? descending : true;
              opts = opts || {};
              this.__expression = expression;
              this.__descending = descending;
              var nulls = isString(opts) ? opts : opts.nulls;
              this.__nulls = isString(nulls) ? nulls.toLowerCase() : null;
          }