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;
}