# first, some sanity check. are all parents valid ids or empty?
for my $node ($self->nodes) {
next if !exists $node->{parent} || !$node->{parent} || $self->{by_id}->{$node->{id}};
# first, some sanity check. are all parents valid ids or empty?
for my $node ($self->nodes) {
next if !exists $node->{parent} || !$node->{parent} || $self->{by_id}->{$node->{id}};