$ bundle

$ gem install literate-programming

First, define the Hello, world!
[[hello-world]] =
  "Hello, world!"

Next, define Main Program; 'Hello, world!'
[[*]] =
  def main
    p [[hello-world]]
  end

Finaly, call the main.
[[*]] +=
  main

First, define the main function and it sings a song ''99 bottles of beer''.
[[*]] =
  def main
    [[bottles:100]]
  end

General case, sing below
If the label-name end with ':@', the rtangle accepts the codes as a general template.
In the general template code,
- if '@'-mark followed by a number, the rtangle replaces it with an argument.
- if '@@'-mark followed by parents, the rtangle replaces it with return value of the insides.
[[bottles:@]] =
  puts '@0 bottles of beer on the wall, @0 bottles of beer.'
  puts 'Take one down and pass it around, @@(@0 - 1) bottles of beer on the wall.'
  [[bottles:@@(@0 - 1)]]

The template code is also able to specialize.
If the rtangle does not find the specialized one,
it specializes from general template code like above.
When number of bottles == 2
[[bottles:2]] =
  puts '2 bottles of beer on the wall, 2 bottles of beer.'
  puts 'Take one down and pass it around, 1 bottle of beer on the wall.'
  [[bottles:1]]

When number of bottles == 1
[[bottles:1]] =
  puts '1 bottle of beer on the wall, 1 bottle of beer.'
  puts 'Take one down and pass it around, no more bottles of beer on the wall.'
  [[bottles:0]]

When no rest bottles...
[[bottles:0]] =
  puts 'No more bottles of beer on the wall, no more bottles of beer.'
  puts 'Go to the store and buy some more, 99 bottles of beer on the wall.'

Finally, call the main function.
[[*]] +=
  main

If you want to do something that requires many sentence,
you can use *before* label;
It will be expanded and be evaluated by rtangle to help to write.
[[*]] =
  def main
    @@(helper_function)
  end

For example, *before* label likes below;
Note: the *before*before* label, the *before*before*before* label, and so on, are also exists.
[[*before*]] =
  def helper_function
    return "p 'Hello, world!'"
  end

Finally, call the main function.
[[*]] +=
  main