準備開始翻譯Antlr的文檔，已經取得了Antlr的作者Terence Parr的授權，將會做為Antlr的中文官方文檔發布。
有興趣一起翻譯的請和我聯系，目前Long、阿樂等人已經加入。有意加入的請首先完成下面的幾個問題，然后Email給我（about521 at 163.com），并告知您的聯系方式等信息：
1、請翻譯下面的段落：
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、編譯器的六個階段：_____、_____、語義分析、中間代碼生成、代碼優化和代碼生成。
3、(1+2)*3的抽象語法樹是什么樣的？