cheat sheets.

if __FILE__ == $0
  ...
end

This is useful in a library file (one containing classes etc. which is
meant to be loaded via *require*, not run directly).

If you want some code to be executed when the file is run directly,
use the idiom above.

In the past, this has been used to put *test* code in a library file,
but these days unit testing is easy enough that such library self-
testing is unnecessary.

However, it's a useful little tool for experimenting with a library in
the early stages of development, without stuffing up any code that
requires the library.

See http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/3431