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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 solve(self, solver_klass=None):
 """ Solves an optimal power flow and returns a results dictionary. """

        # Start the clock.
        t0 = time()

        # Build an OPF model with variables and constraints.
        om = self._construct_opf_model(self.case)
        if om is None:
            return {"converged": False, "output": {"message": "No Ref Bus."}}

        # Call the specific solver.
#        if self.opt["verbose"]:
#            print '\nPYLON Version %s, %s', "0.4.2", "April 2010"
        if solver_klass is not None:
            result = solver_klass(om, opt=self.opt).solve()
        elif self.dc:
#            if self.opt["verbose"]:
#                print ' -- DC Optimal Power Flow\n'
            result = DCOPFSolver(om, opt=self.opt).solve()
        else:
#            if self.opt["verbose"]:
#                print ' -- AC Optimal Power Flow\n'
            result = PIPSSolver(om, opt=self.opt).solve()

        result["elapsed"] = time() - t0

        if self.opt.has_key("verbose"):
            if self.opt["verbose"]:
                logger.info("OPF completed in %.3fs." % result["elapsed"])

        return result