In: Lecture Notes in Computer Science : Developments in Language Theory, 2006, pages 351-362. 2006. URL: http://dx.doi.org/10.1007/1177914832.
Abstract: The class of semi-rational dag languages can be characterized by labeled Petri nets with e-transitions, by rather simple leaf substituting tree grammars with additional non-local merge rules, or as a synchronization closure of Courcelles class of recognizable sets of unranked, unordered trees. However, no direct recognition by some magma is known. For a better understanding, we present here some examples of languages within and without the class of semi-rational dag languages.