Dora Giammarresi Antonio Restivo Ambiguity and Complementation in Recognizable Two-dimensional Languages. 5-20 2008 IFIP TCS http://dx.doi.org/10.1007/978-0-387-09680-3_1 conf/ifipTCS/2008 db/conf/ifipTCS/ifipTCS2008.html#GiammarresiR08