function select_all(arr){
MyArray = arr;
    for(i=0;i<MyArray.length;i++){
        document.getElementById(MyArray[i]).checked = true;
    }
}
function deselect_all(arr){
MyArray = arr;
    for(i=0;i<MyArray.length;i++){
        document.getElementById(MyArray[i]).checked = false;
    }
}