features with no ID attribute but with Parent attributes

[billzt]

the script assumes, incorrectly, that all items with a Parent have an ID, which is not required (unless they themselves have children), and such subfeatures would not be placed by the script

push@{$parent2children{$parent} }, $id #when $id is void?

Thanks to Miklos Csuros’ review
In fact, the current code is able to deal with this.

if (defined($id)) {
     $id2line{$id} = $line;
}
else {
     $id2line{$line} = $line;
}

Considering to add some comments here.