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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 _nln_constraints(self, nb, nl):
 """ Returns non-linear constraints for OPF. """

        Pmis = NonLinearConstraint("Pmis", nb)
        Qmis = NonLinearConstraint("Qmis", nb)
        Sf = NonLinearConstraint("Sf", nl)
        St = NonLinearConstraint("St", nl)

        return Pmis, Qmis, Sf, St