Reported by michael.prilop @tmql4J

We need a fn:best-identifier which delivers a stable identifier for a given topic.

best-identifier should be in following ordering a 1. subject identifier (si) 2. subject locator (sl) 3. item identifier (ii) if several identifier of the same type are present the shortest should be choosen (e.g. there are two si -> choose shortes si regardless of sl or ii). The returned identifier should be prefixed with its type e.g. si:http://example.com/something

see:

http://code.google.com/p/tmql/issues/detail?id=25