function command( pres ) {
	return onCommand;

	/**
	* Returns the presentation length.
	*
	* @private
	* @returns {integer} presentation length
	*/
	function onCommand() {
		return pres.length;
	}
}
Returns a callback to be invoked upon calling the `numSlides` command.

@private
@param {Presentation} pres - presentation instance
@returns {Function} callback