Metaprogramming koans

I found more koans for expanding awareness. Compared to callcc koans, these are much simpler, except for trick with evaluating default block argument in instance context.

It seems that most people did it Scheme way (with hygienic macros analogue), but I followed the Common Lisp path to enlightenment with gensym and $gensym_counter ^^; That was pure fun evil. Of course binding variables in lexical context is almost always better than gensyming in Ruby.