# Compare two elements using either standard lexical comparison or the sortsub
# provided to the object's constructor.
sub _compare {
    my ( $self, $item_a, $item_b ) = @_;
    my $sortsub = $self->_get_sortsub;
    if ( defined $sortsub ) {
        local $a = $item_a;
        local $b = $item_b;
        return $sortsub->( $a, $b );
    }
    else {
        return $item_a cmp $item_b;
    }
}