sub NEXTKEY {
  my $self    = shift;
  #my $lastkey = shift;  # Do you need this?  If not, don't bother shifting it

  until($self->{FIRSTKEY} eq $KEYS[$self->{INDEX}]) {
    ++$self->{INDEX};
    last if $self->{INDEX} >= $#KEYS;  # Quit if we run out of @KEYS
  }

  return($KEYS[$self->{INDEX}]);
}