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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.