activeSheet() {
        return new ArgHandler('Workbook.activeSheet')
            .case(() => {
                return this._activeSheet;
            })
            .case('*', sheet => {
                // Get the sheet from name/index if needed.
                if (!(sheet instanceof Sheet)) sheet = this.sheet(sheet);

                // Check if the sheet is hidden.
                if (sheet.hidden()) throw new Error("You may not activate a hidden sheet.");

                // Deselect all sheets except the active one (mirroring ying Excel behavior).
                _.forEach(this._sheets, current => {
                    current.tabSelected(current === sheet);
                });

                this._activeSheet = sheet;

                return this;
            })
            .handle(arguments);
    }
Set the active sheet in the workbook.
@param {Sheet|string|number} sheet - The sheet or name of sheet or index of sheet to activate. The sheet must not be hidden.
@returns {Workbook} The workbook.