function validate_form(f) {

  /** The following two lines avoid some crazy DOM problem in IE7. **/
  var x = f.adult.value;
  var y = f.youth.value;
  /** It appears that the two declarations which follow fail when the fields are
      manipulated to change field type, etc.
  **/

  var adult = f.adult;
  var youth = f.youth;

  var qty = 0;

  if (adult) {
    if (isNaN(adult.value)) {
      alert("Please specify a valid quantity.");
      return false;
    }
    qty += adult.value;
  }
 

  if (youth) {
    if (isNaN(youth.value)) {
      alert("Please specify a valid quantity.");
      return false;
    }
    qty += youth.value;
  }

  if (qty == 0) {
    alert("Please specify a valid quantity.");
    return false;
  }

  return true;
}