Top-Down Parsing
自顶向下分析（top-down parsing） 就是从起始符号
由于展开有多重可能性，所以只有推导出所有的句子
主要实现：
Wikipedia
Top-down parsing in computer science is a parsing strategy where one first looks at the highest level of the parse tree and works down the parse tree by using the rewriting rules of a formal grammar. LL parsers are a type of parser that uses a top-down parsing strategy.[1]