1. Lecture Notes in Computer Science;B Bollig,2009
2. Brüggemann-Klein, A., Murata, M., Wood, D.: Regular tree and regular hedge languages over unranked alphabets: version 1. Technical Report HKUST-TCSC-2001-0, The Honkong University of Sience and Technologie (2001)
3. Brüggemann-Klein, A., Wood, D.: Regular tree languages over non-ranked alphabets (1998). http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.50.5397
4. Lecture Notes in Computer Science;K Chatterjee,2008
5. Lecture Notes in Computer Science;K Chatterjee,2009