code: func (cs *Constraints) Set(str string) error {
  	exps := strings.Split(str, ",")
  	if len(exps) == 0 {
  		return fmt.Errorf("bad Constraint format: %s", str)
  	}
  	for _, exp := range exps {
  		constraint, err := NewConstraint(exp)
  		if err != nil {
  			return err
  		}
  		*cs = append(*cs, constraint)
  	}
  	return nil
  }
docs: // Set []*Constraint.