code: func tokenConstraintString(name string, value ast.Value, where *string) (err error) {
  	var av ast.String
  	var ok bool
  	if av, ok = value.(ast.String); !ok {
  		err = fmt.Errorf("%s constraint: must be a string", name)
  		return
  	}
  	*where = string(av)
  	return
  }
docs: // tokenConstraintString handles string constraints.