code: func (s ConstraintStr) Parse() (Constraints, error) {
  	raw, err := version.NewConstraint(string(s))
  	if err != nil {
  		return Constraints{}, err
  	}
  	return Constraints{raw}, nil
  }
docs: // Parse transforms a ConstraintStr into a Constraints if it is
  // syntactically valid. If it isn't then an error is returned instead.