var checked=false;

//vybere vsechny checkboxy se jmenem checkboxName a vybere/odvybere je
function checkAll (checkboxName) {
	var checkboxes = document.getElementsByName(checkboxName);
	if (checked == false) {
		checked = true
    }
    else {
		checked = false
	}
	for (var i =0; i < checkboxes.length; i++){
		checkboxes[i].checked = checked;
	}
}