var checkflag = "false";
function check(field) {
if (checkflag == "false") {
  for (i = 0; i < field.length; i++) {
  field[i].checked = true;}
  checkflag = "true";
  return "Uncheck all"; }
else {
  for (i = 0; i < field.length; i++) {
  field[i].checked = false; }
  checkflag = "false";
  return "Check all"; }
}