code: func StringOperand(x ast.Value, pos int) (ast.String, error) {
  	s, ok := x.(ast.String)
  	if !ok {
  		return ast.String(""), NewOperandTypeErr(pos, x, "string")
  	}
  	return s, nil
  }
docs: // StringOperand converts x to a string. If the cast fails, a descriptive error is
  // returned.