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### Remarks
Just run it with no argument:
ruby entry.rb
(Anyway it is just a no-op program. The above command only verifies
that entry.rb is a valid Ruby program.)
I confirmed the following implementations/platforms:
* ruby 2.5.0p0 (2017-12-25 revision 61468) [x64-mingw32]
### Description
First, look at
https://docs.ruby-lang.org/ja/latest/doc/spec=2flexical.html#reserved
and then, look at entry.rb.
The source code of entry.rb consists only of reserved words of Ruby,
and all the reserved words are used in the code, in a way that the code
forms a valid Ruby program. No compile error, no warning, or no runtime error.
### Internals
Difficult (and interesting) points of the theme are:
* Since many of the reserved words define program structures, we cannot
use them independently. For instance, `retry` must be inside `rescue`,
or `break`/`next`/`redo` must be inside a looping construct.
Or, jump-out statements cannot occur at a position that requires a
value; `if return then true end` is a "void value expression" syntax error.
* Inserting newlines for each 6 word (to match with the spec html) is also
an interseting challenge, since Ruby is sensitive to newlines.
Tricks used in the code are:
* def/alias/undef can take even reserved words as parameters.
That is, `def class ... end` defines a method named `class`.
The feature is crucial since otherwise `BEGIN` etc inevitably
introduces non-reserved tokens (like `{}`).
* `defined?` can take some reserved words too (which I didn't know
until trying to write this program.)
* "void value expression" can be avoided by using `or` or `and`.
`if begin return end then true end` is a syntax error, but
`if begin false or return end then true end` is not.
### Limitation
Sad to say that it's not a "perfect pangram".
It uses 'alias' and 'undef' twice, and 'end' 4 times.
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