code: def get_constraint(self, twig=None, **kwargs):
          
          if twig is not None:
              kwargs['twig'] = twig
          kwargs['context'] = 'constraint'
          return self.get(**kwargs)
docs: Filter in the 'constraint' context
  
          :parameter str constraint: name of the constraint (optional)
          :parameter **kwargs: any other tags to do the filter
              (except constraint or context)
          :return: :class:`phoebe.parameters.parameters.ParameterSet`