Am I true when expecting that every item has zero or one direct predecessor (in the 'quasiorder' meaning) ?

If this is true, it seems to be easy to read the sequences and build the structure of kind

{$name => [$count_of_my_predecessorship, $my_direct_predecessor_name]}

, and after end of building to check count of predecessorship for each key. Count == 0 => this is root, count == 1 => common item, count > 1 => anounced ambiguity. If you have huge data, it will not be so easy :>)

Update: Count == 0 => this is leave

Update2: Count == 0 => this is leaf, sorry for my English :>)