Erik, when I've needed set-like behavior in ruby, I just used the Set 
class. :-) You should see if a set of hashes does what you need.

As far as having a .to_h method for Nil, I think you've got a good idea 
there. Nil has to_a, to_c, to_f, to_i, to_r, and to_s; having a to_h 
seems consistent and expected to me. I recommend you write a patch and 
submit it to the ruby developers. Even if C isn't your language, you 
should be able to figure out something this simple by looking at the 
source for similar methods.
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