Necessity of timing dependency in parallel programming languages

There are two classes of dataflow schemata: DF and ADF. ADF is known to be equivalent to EF and DF, and the class of ordinary dataflow schemata is known to be equivalent to EF/sup d/. ADF is given by strengthening with two devices compared with DF, the class of ordinary dataflow schemata. One is recursion and the other is arbiter which allows timing dependent processing. We are interested in whether both devices are necessary for ADF to have such powerful expression ability. The author examines the expression ability of the class RDF which is strengthened with just recursion compared to DF. As a result it is shown that RDF is also equivalent to EF/sup d/, which means that some kind of timing dependency is necessary for the class of dataflow schemata to be powerful enough.