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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def formulas_iter(self):
 """ Iterates over all variable-clauses in the logical network Yields ------ tuple[str,frozenset[caspo.core.clause.Clause]] The next tuple of the form (variable, set of clauses) in the logical network. """

        for var in it.ifilter(self.has_node, self.variables()):
            yield var, frozenset(self.predecessors(var))