Alberto Bertoni Massimiliano Goldwurm Violetta Lonati The Complexity of Unary Tiling Recognizable Picture Languages: Nondeterministic and Unambiguous Cases. 231-249 2009 91 Fundam. Inform. 2 http://dx.doi.org/10.3233/FI-2009-0042 db/journals/fuin/fuin91.html#BertoniGL09