Andrzej Ehrenfeucht Tero Harju Grzegorz Rozenberg Theory of 2-Structures. 1-14 1995 conf/icalp/1995 ICALP db/conf/icalp/icalp95.html#EhrenfeuchtHR95 http://dx.doi.org/10.1007/3-540-60084-1_58