merged() {
        return new ArgHandler('Sheet.merge')
            .case('string', address => {
                return this._mergeCells.hasOwnProperty(address);
            })
            .case(['string', '*'], (address, merge) => {
                if (merge) {
                    this._mergeCells[address] = {
                        name: 'mergeCell',
                        attributes: { ref: address },
                        children: []
                    };
                } else {
                    delete this._mergeCells[address];
                }

                return this;
            })
            .handle(arguments);
    }
Merge/unmerge cells by adding/removing a mergeCell entry.
@param {string} address - The address to merge.
@param {boolean} merged - True to merge, false to unmerge.
@returns {Sheet} The sheet.
@ignore