code: def _ref_bus_angle_constraint(self, buses, Va, xmin, xmax):
          
          refs = [bus._i for bus in buses if bus.type == REFERENCE]
          Varefs = array([b.v_angle for b in buses if b.type == REFERENCE])
  
          xmin[Va.i1 - 1 + refs] = Varefs
          xmax[Va.iN - 1 + refs] = Varefs
  
          return xmin, xmax
docs: Adds a constraint on the reference bus angles.