It works after I added 'static' to the declaration of 'invoke()'
(and import std.stdio, std.traits.)
I fiddled around with it for hours before I tried 'static' there,
because I've only been studying D for a week, so only about half
of this code and the error messages made any sense to me when I
started on it.
Once it started passing all the tests, I still didn't quite get
how it all worked, because of suspecting there was a typo in the
code between writing TestClass and testClass. So I tested whether
specific instances are called (instead of maybe just the first
one constructed) by adding a static counter to TestClass, and
storage of the counter by each instance. It looks all right.
