sub find_path {
  my ($start_at, $end_at, $been_there) = (@_, {});
  if ($start_at == $end_at) {
    return [$start_at];
  }
  else {
    my @try_these = grep { ! $been_there->{$_} } @{$adjacency_list{$start_at}};
    return map {
      my @cdr_list = find_path($_, $end_at, {%$been_there, $start_at => 1});
      map [$start_at, @$_], @cdr_list;
    } @try_these;
  }
}