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

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

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

$node->set_taxon( $taxon );

sub set_taxon {
    my ( $node, $taxon ) = @_;
...
    $taxon->add_node( $node );
    return $node;
}