/**

 * Return TRUE if the given function has been defined.

 */

function function_exists (function_name) {

	// Original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)

	if (typeof function_name == 'string'){

		return (typeof this.window[function_name] == 'function');

	} else{

		return (function_name instanceof Function);

	}

}