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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 derived_solutions(self, solution=None):
 """Returns all possible solutions based on the provide solution. This assumes that the given solution is incomplete. """

        solution = solution or Solution()
        used_variables = solution.keys()
        unused_variables = (v for v in self._variable_expand_order if v not in used_variables)
        for var in unused_variables:
            for value in self._vars[var]:
                yield Solution({var: value}, solution)