sub find_solution {
  my ($state,
      $fitness_fcn,
      $nextmoves_fcn,
      $applymove_fcn,
     ) = @_;
  return () unless $fitness_fcn->($state);
  my @next_moves = $nextmoves_fcn->($state);
  return $state unless @next_moves;
  return map {
    find_solution($applymove_fcn->($state, $_),
                  $fitness_fcn,
                  $nextmoves_fcn,
                  $applymove_fcn,
                 );
  } grep {
    $fitness_fcn->($applymove_fcn->($state, $_);
  } @next_moves;
}