function enableSelect (num_id) {

  for (var i=1; i<=7; ++i) {
    var elemName = "sel_area_" + i
    var elem = document.getElementById(elemName);
    elem.disabled = true;
  }

  if (num_id) {
    var elemName = "sel_area_" + num_id;
    var elem = document.getElementById(elemName);
    elem.disabled = false;
  }
}