sub add_node {
  my $self = shift;
  my $node = shift;
  return if is_duplicate( $node ); # do nothing for dups
  # proceed to add node to the set of nodes for this tree
}

##</code><code>##

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

##</code><code>##

$tree->add_node( Node->new( $some_random_object, $id ) );

##</code><code>##

sub { my $node = shift; return "$node" }

##</code><code>##

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 );
}

##</code><code>##

sub get_id {
  my $self = shift;
  my $node = shift;  # $node->isa( 'Node' )
  return
    ref $node && $node->can( 'id' ) ? $node->id
                                    : $self->id_fxn->( $node );
}