编译原理中实现自顶向下语法分析,若文法中不含左递归,并且每个非终结符的所有后选式的首符集都两两不相交,那么就有可能构造一个不带回溯的自顶向下的语法分析程序-Compiler theory to achieve top-down parsing　When every not terminal goes without interacting with each other,It can make a process of form up to down