准备开始翻译Antlr的文档，已经取得了Antlr的作者Terence Parr的授权，将会做为Antlr的中文官方文档发布。
      有兴趣一起翻译的请和我联系，目前Long、阿乐等人已经加入。有意加入的请首先完成下面的几个问题，然后Email给我（about521 at 163.com），并告知您的联系方式等信息：
1、请翻译下面的段落：
A lexer (often called a scanner) breaks up an input stream of characters into vocabulary symbols for a parser, which applies a grammatical structure to that symbol stream. Because ANTLR employs the same recognition mechanism for lexing, parsing, and tree parsing, ANTLR-generated lexers are much stronger than DFA-based lexers such as those generated by DLG (from PCCTS 1.33) and lex.
2、编译器的六个阶段：_____、_____、语义分析、中间代码生成、代码优化和代码生成。
3、(1+2)*3的抽象语法树是什么样的？