code: func selectConstant(tpe types.Type, term *Term) types.Type {
  	x, err := JSON(term.Value)
  	if err == nil {
  		return types.Select(tpe, x)
  	}
  	return nil
  }
docs: // selectConstant returns the attribute of the type referred to by the term. If
  // the attribute type cannot be determined, nil is returned.