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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 describe_variables(self, v):
 """ Given an AST, iterate over all the keys of all the BVS leaves in the tree which are registered. """

        reverse_mapping = {next(iter(var.variables)): k for k, var in self.eternal_tracked_variables.items()}
        reverse_mapping.update({next(iter(var.variables)): k for k, var in self.temporal_tracked_variables.items() if k[-1] is not None})

        for var in v.variables:
            if var in reverse_mapping:
                yield reverse_mapping[var]