code: def run_constraint(self, twig=None, return_parameter=False, **kwargs):
          
          if not kwargs.get('skip_kwargs_checks', False):
              self._kwargs_checks(kwargs)
  
          kwargs['twig'] = twig
          kwargs['context'] = 'constraint'
          # kwargs['qualifier'] = 'expression'
          kwargs['check_visible'] = False
          kwargs['check_default'] = False
          # print "***", kwargs
          expression_param = self.get_parameter(**kwargs)
  
          kwargs = {}
          kwargs['twig'] = None
          # TODO: this might not be the case, we just know its not in constraint
          kwargs['qualifier'] = expression_param.qualifier
          kwargs['component'] = expression_param.component
          kwargs['dataset'] = expression_param.dataset
          kwargs['feature'] = expression_param.feature
          kwargs['context'] = []
          if kwargs['component'] is not None:
              kwargs['context'] += ['component']
          if kwargs['dataset'] is not None:
              kwargs['context'] += ['dataset']
          if kwargs['feature'] is not None:
              kwargs['context'] += ['feature']
  
          kwargs['check_visible'] = False
          kwargs['check_default'] = False
          constrained_param = self.get_parameter(**kwargs)
  
          result = expression_param.result
  
          constrained_param.set_value(result, force=True, run_constraints=True)
  
          logger.debug("setting '{}'={} from '{}' constraint".format(constrained_param.uniquetwig, result, expression_param.uniquetwig))
  
          if return_parameter:
              return constrained_param
          else:
              return result
docs: Run a given 'constraint' now and set the value of the constrained
          parameter.  In general, there shouldn't be any need to manually
          call this - constraints should automatically be run whenever a
          dependent parameter's value is change.
  
          :parameter str twig: twig to filter for the constraint
          :parameter **kwargs: any other tags to do the filter
              (except twig or context)
          :return: the resulting value of the constraint
          :rtype: float or units.Quantity