One-to-many:
taxon ---> node
      ---> node
      ---> sequence
      ---> sequence

One-to-one:
node ---> taxon

One-to-one:
sequence ---> taxon

##</code><code>##

my @nodes = $taxon->get_nodes;
my @sequences = $taxon->get_sequences;
my $taxon = $node->get_taxon;
$taxon = $sequence->get_taxon;

##</code><code>##

$node->set_taxon( $taxon );

##</code><code>##

sub set_taxon {
    my ( $node, $taxon ) = @_;
    $node->{'_taxon'} = $taxon;
    # check if taxon already knows about me
    for my $known ( $taxon->get_nodes ) {
        return $node if $known == $node;
    }
    $taxon->add_node( $node );
    return $node;
}