TreeNode * stmt_sequence(void)
{
     TreeNode * t = statement();
     TreeNode * p = t;
     while ((token!=ENDFILE) && (token!=END) &&
           (token!=ELSE) && (token!=UNTIL))
/*
 * if-stmt -> if exp then stmt-sequence [ else stmt-sequence ] end
 * 可见stmt-sequence后面可能跟else ， end ，然而这并不是 stmt_sequence 函数  *  该管的事，须由if_stmt函数来检测
 * 所以 有 (token!=END) && (token!=ELSE)，其它 （ (token!=UNTIL) ） 同理
 */
     {
          TreeNode * q;
          match(SEMI);
          q = statement();
          if (q!=NULL) {
               if (t==NULL) t = p = q;
               else /* now p cannot be NULL either */
               {
                    p->sibling = q;
                    p = q;
               }
          }
     }
     return t;
}