之前 Google 的 nofollow 用于 <meta> 标签中，如：
<meta name="robots" content="nofollow">
表示拒绝 robot 跟踪本页链接，但可索引本页。而如今已经可以细化到每一个链接了，即在链接标签 <a> 中直接使用 nofollow 属性，如：
<a href="http://www.site.com/page.html" rel="nofollow">Visit My Page</a>
使用了 nofollow 属性之后意味着什么呢？
1) Robot 不会跟踪此链接地址；
2) 不会对此链接地址进行 PageRank 计算；
3) 此链接标签所包含的文字也不会作为检索的关联词。
Google 想通过这种方式来阻止 SPAM，例如，通过在邮件地址或 BLOG 评论的链接标签中加入 onfollow，可以有效地向 robot 表明不要对相关链接地址进行跟踪、索引，虽然还不知道这个 nofollow 属性是否对 SPAM 有效，但至少 Google 给了页面所有者选择是否对链接地址进行索引的权利。
参考资料：
Google, Yahoo, MSN Unite On Support For Nofollow Attribute For Links
Google To Add "Nofollow" Tagging Of Links To Fight Spam?
一本不错的在线书籍：Multithreading - The Delphi Way，作者 [i]Martin Harvey[/i] 对 Delphi 的 MultiThread 表述得很通俗易懂。此书适合有 Delphi 面向对象编程基础的读者。Introduction.
This guide is intended for anyone who is interested in improving performance and responsiveness in their Delphi applications by using threads. It covers a range of topics from absolute beginner to intermediate level, and some of the real world examples raise issues bordering on the advanced. It assumes that the reader has a reasonable knowledge of Object Pascal programming, including simple object orientation, and a working understanding of event based programming.
Table of Contents.
Introduction.
Dedications.
Recommended Reading.
Navigation hints.
Change history.
Credits.
Chapter 1. What are threads? Why use them?
Chapter 2. Creating a thread in Delphi.
Chapter 3. Basic synchronization.
Chapter 4. Simple thread destruction.
Chapter 5. More thread destruction. Deadlock.
Chapter 6. More synchronization: Critical sections and mutexes.
Chapter 7. Mutex programming guidelines. Concurrency control.
Chapter 8. Delphi thread safe classes and Priorities.
Chapter 9. Semaphores. Data flow scheduling. The producer - consumer relationship.
Chapter 10. I/O and data flow: from blocking to asynchronous and back.
Chapter 11. Synchronizers and Events.
Chapter 12. More Win32 synchronization facilities.
Chapter 13. Using threads in conjunction with the BDE, Exceptions and DLLs.
Chapter 14. A real world problem, and its solution.
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OOP and PerformanceObject oriented programming, compared to procedural is typically seen as a trade off: increased "developer performance" through better modularity / re-use vs. slower processing, based on the extra runtime lookup overhead objects introduce (compared to the equivalent collection of functions + variables).
