code: def _nln_constraints(self, nb, nl):
          
          Pmis = NonLinearConstraint("Pmis", nb)
          Qmis = NonLinearConstraint("Qmis", nb)
          Sf = NonLinearConstraint("Sf", nl)
          St = NonLinearConstraint("St", nl)
  
          return Pmis, Qmis, Sf, St
docs: Returns non-linear constraints for OPF.