sub add_node {
  my $self = shift;
  my $node = shift;
  my $id = get_id( $node );
  return $self->{ N }{ $id } ||= $node;
}

sub get_id {
  my $self = shift;
  my $node = shift;
  my $id_fxn = $self->id_fxn;
  return $id_fxn( $node );
}