sub find_once
{
  my ( $str , ... ) = @_ ;
  ...
  my ( $pos , $once ) = ( 0 ) x2;
  while ( my $p = get_permutation_string() )
  {
    last if $once > 1;
    ( $pos = index $str , $p , $pos ) >= 0 and $once++ ;
  }
  return $once == 1;
}