document: P systems with symport/antiport rules : the traces of objects We continue the study of those P systems where the computation is performed by the communication of objects , that is , systems with symport and antiport rules . Instead of the -LRB- number of -RRB- objects collected in a specified membrane , as the result of a computation we consider the itineraries of a certain object through membranes , during a halting computation , written as a coding of the string of labels of the visited membranes . The family of languages generated in this way is investigated with respect to its place in the Chomsky hierarchy . When the -LRB- symport and antiport -RRB- rules are applied in a conditional manner , promoted or inhibited by certain objects which should be present in the membrane where a rule is applied , then a characterization of recursively enumerable languages is obtained ; the power of systems with the rules applied freely is only partially described
doc_bio_tags: B I O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O