* The previous convention of !* ... *! are now true block comments,
i.e. they are parsed faster, don't spam the goto table and allow
embedding of exclamation marks - only "*!" terminates the comment.
* It is therefore now forbidden to have goto labels beginning with "*".
* Also support "!!" to introduce EOL comments (like C++'s //).
This disallows empty labels, but they weren't useful anyway.
This is the shortest way to begin a comment.
* All comment labels have been converted to true comments, to ensure
that syntax highlighting works correctly.
EOL comments are used for single line commented-out code, since it's
easiest to uncomment - you don't have to jump to the line end.
This is a pure convention / coding style.
Other people might do it differently.
* It's of course still possible to abuse goto labels as comments
as TECO did for ages.
* In lexing / syntax highlighting, labels and comments are highlighted differently.
* When syntax highlighting, a single "!" will first be highlighted as a label
since it's not yet unambiguous. Once you type the second character (* or !),
the first character is retroactively styled as a comment as well.