sub leaves_under {
  # read-only method:  return a list of all leaves under myself.
  # Returns myself in the degenerate case of being a leaf myself.
  my $node = shift;
  my @List = ();
  # $node->no_cyclicity;
  $node->walk_down({ 'callback' =>
    sub {
      my $node = $_[0];
      my @daughters = @{$node->{'daughters'}};
      push(@List, $node) unless @daughters;
      return 1;
    }
  });
  Carp::croak "Spork Error 861: \@List has no contents!?!?" unless @List;
    # impossible
  return @List;
}