# Small and large categories

A category C is called small if both ob(C) and hom(C) are actually sets and not proper classes, and large otherwise. A locally small category is a category such that for all objects a and b, the hom-class hom(a, b) is a set, called a homset. Many important categories in mathematics (such as the category of sets), although not small, are at least locally small.

— Wikipedia on Category (mathematics)

2010.03.31 Wednesday ACHK

# Start a company

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When we asked the summer founders what surprised them most about starting a company, one said “the most shocking thing is that it worked.”

— Paul Graham

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2010.03.31 Wednesday $ACHK$

# OCD 4

Past papers 14

“Past papers” means “past HKCEE/HKAL examination papers”. The topic is for Hong Kong students who are facing HKCEE or HKAL. But the general principles can also be used for tackling other public examinations.

（KYH：我考試有時明明某一題做完做對了，但是覺得寫得很混亂，所以會擦掉，重新寫一次。）

— Me@2010.03.30

# Lisp macros

Lisp’s uniform, parenthesized syntax works especially well with macros. Languages of the Lisp family, such as Common Lisp and Scheme, have powerful macro systems because the syntax is simple enough to be parsed easily. Lisp macros transform the program structure itself, with the full language available to express such transformations. Common Lisp and Scheme differ in their macro systems: Scheme’s is based on pattern matching, while Common Lisp macros are functions that explicitly construct sections of the program.

Being able to choose the order of evaluation (see lazy evaluation and non-strict functions) enables the creation of new syntactic constructs (e.g. control structures) indistinguishable from those built into the language. For instance, in a Lisp dialect that has cond but lacks if, it is possible to define the latter in terms of the former using macros.

Macros also make it possible to define data languages that are immediately compiled into code, which means that constructs such as state machines can be implemented in a way that is both natural and efficient.

— Wikipedia on Macro (computer science)

2010.03.30 Tuesday ACHK

# 原始時代

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— Me@2010.03.29

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2010.03.30 Tuesday $copyright ACHK$

# 多次元宇宙 7

（安：暫時也不是很明白。除了說「即是」之外，《心經》還說「不異」：「色不異空，空不異色」。

《心經》說「色『即是』空」，意思是：

「本體」是不能直接被觀察到的，因為我們不能『直接看到』「本體」。我們只能透過「現象」去觀察「本體」。「透過『現象』去觀察『本體』」是「觀察『本體』」的唯一方法。

「色即是空，空即是色」的意思是，雖然「現象」和「本體」是不同的東西，但是，要觀察「本體」，我們只能透過「現象」去觀察「本體」。正如，雖然「杯的影像」和「杯本身」是不同的東西，但是，要觀察「杯本身」，我們只能透過「杯的影像」去觀察「杯本身」。

「色即是空，空即是色」中的「即是」，是指當我們說

「我的眼睛接收到一隻『杯』的『影像』。」時，意思即是

「我看到一隻『杯』。」；

「我看到一隻『杯』。」時，意思又即是

「我的眼睛接收到一隻『杯』的『影像』。」

— Me@2010.03.29

# Passport

Mainland China and Taiwan

The People’s Republic of China (PRC) and the Republic of China (ROC) do not recognise each other as sovereign states. They both claim themselves as the only legal government representing the whole China.

Consistent with the 1992 Consensus, the PRC and ROC legally consider both citizens in mainland China and Taiwan as their own citizens, but residing in different areas of the same nation. Neither the PRC nor the ROC accepts passports issued by the other as entry documents.

Citizens in Taiwan use identity documents issued by PRC public-security authorities to enter mainland China. Citizens in mainland China entering Taiwan must also use identity documents issued by the ROC authority, and have their mainland documents surrendered. The identity documents cannot be used for international travel, and an endorsement must be obtained separately to enable travel.

The ROC used to require its citizens who intended travel to mainland China to obtain official approval for the travel, and prescribed an administrative fine of NT\$20,000 to NT\$100,000 for those who did not. However, the fine was often unenforceable because such travel was untraceable by examination of travel documents, except if an ROC citizen lost his ROC passport while on the mainland, and, so, had to report the loss. The official-approval requirement was abolished, except in relation to ROC officials, of whom applications are required.

— Wikipedia on Passport

2010.03.29 Monday ACHK

# 心生 3

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What you are

comes to you.

– Ralph Waldo Emerson

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2010.03.29 Monday $ACHK$

# Clebsch-Gordan coefficients software

This program is written by me to calculate the Clebsch-Gordan coefficients.

To use it, copy-and-paste the text of this file onto DrScheme and run it.

Within DrScheme, you have to select the Language “Pretty Big”.

You can contact me if you find any bugs in this program.

# Batman Begins

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Blogger Mark Fisher states that Bruce’s search for justice requires him to learn from a proper father figure, with Thomas Wayne and Ra’s al Ghul being the two counterpoints. Alfred provides a maternal figure of unconditional love, despite the overall lack of focus on a mother figure in Bruce’s life.

Impact

Shawn Adler of MTV stated after Batman Begins, a trend of darker genre films, that either retold back-stories or rebooted them altogether, began.
Examples he cited were Casino Royale, as well as the in-development RoboCop, Red Sonja, and He-Man. Filmmakers, screenwriters and producers who have mentioned the film to describe their projects include: Jon Favreau and Iron Man, Edward Norton and The Incredible Hulk, McG and Terminator Salvation (which also stars Bale), Damon Lindelof and Star Trek, Robert Downey, Jr. and Sherlock Holmes, Lorenzo di Bonaventura and G.I. Joe: The Rise of Cobra, and even Hugh Jackman and X-Men Origins: Wolverine.

— Wikipedia on Batman Begins

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2010.03.28 Sunday $ACHK$

# Copyleft 4

The Open Invention Network (OIN) is a company that acquires patents and licenses them royalty free to entities which, in turn, agree not to assert their own patents against Linux or Linux-related applications.

— Wikipedia on Open Invention Network

2010.03.27 Saturday ACHK

# 浮水 2

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– Diary, Me@2000.04.28

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2010.03.27 Saturday $copyright ACHK$

# OCD 3

Past papers 13

“Past papers” means “past HKCEE/HKAL examination papers”. The topic is for Hong Kong students who are facing HKCEE or HKAL. But the general principles can also be used for tackling other public examinations.

（KYH：我的是另外一個類似的問題。我有時寫了一東西，例如計完一題數，我覺得字體不夠好，會擦掉，重新寫一次。那算不算是情緒病？）

（KYH：OK 啦 …）

「OK」即是「頗為嚴重」？

（KYH：我也不知道算不算嚴重。）

「嚴重」的意思是，你的問題，除了令你心裡不舒服外，對你還有「實質影響」。你的問題對你有沒有「實質影響」？

（KYH：我考試有時明明某一題做完做對了，但是覺得寫得很混亂，所以會擦掉，重新寫一次。）

— Me@2010.03.26

# The Maxwell’s equations of programming

Interactive means that programming is a dialog with Lisp. You enter an expression and Lisp computes the side effects (for example output) and the value.

So your programming session is like ‘talking’ with the Lisp system. You work with it until you get the right answers.

Alan Kay (of Smalltalk fame) calls the original definition of Lisp evaluation in Lisp the Maxwell’s equations of programming.

— Rainer Joswig

2010.03.26 Friday ACHK

# Be rewritten

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Another group was worried when they realized they had to rewrite their software from scratch. I told them it would be a bad sign if they didn’t. The main function of your initial version is to be rewritten.

— Paul Graham

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2010.03.26 Friday $ACHK$

# 50 50

— Me@around 2000

— Me, based on Mr Lee@around 2007-2008

— an updated version by Me@around 2007-2008

— Me@2010.03.25

# Recursion 2

To iterate is human; to recurse, divine.

L. Peter Deutsch

2010.03.25 Thursday ACHK

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The way to get an academic position is to write great books, proving your academic competence. But you may not want an university-academic life.

— Me@around 2007

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The way to get an academic position is not to get a PhD first, but to write great books, proving your academic competence. But you may not want an university-academic life.

2010.03.25 Thursday $copyright ACHK$

# 天才省略

Past papers 12

“Past papers” means “past HKCEE/HKAL examination papers”. The topic is for Hong Kong students who are facing HKCEE or HKAL. But the general principles can also be used for tackling other public examinations.

The art of being wise is the art of knowing what to overlook. –- William James

「天才」，就是知道什麼可以省略的人。但是要「知道什麼可以省略」，本身就要有強大的背景知識，一般人也沒有。所以如果你是「一般人」，又要「知道什麼可以省略」的話，你可以用以下兩個捷徑：

— Me@2010.03.24