the class definitions of my ruby file shall always be active, whether
the file is run as standalone program or whether the file is used
indirectly by require or load,
but when the file is run standalone, a special piece of code within
the file shall be active in addition (a kind of main section).
The idea is that the same ruby code file should be able to act in both
kinds: as a library and as a standalone program which is using that
How can I achieve that easily?