code: public BeetlException validate(){
  		if(!(program instanceof ErrorGrammarProgram)){
  			return null;
  		}
  		ErrorGrammarProgram error = (ErrorGrammarProgram)program;
  		BeetlException exception = error.getException();
  		return exception;
  	}
docs: 语法校验，如果返回BeetlException，则表示语法有错，返回null，语法无错误
  @return