Thanks Rick. I poked around a bit more after the last few posts made 
here and couldn't find an answer. I started looking at the Ruby's C code 
but couldn't really get any useful information from it. Despite this I'm 
fairly confident what I wanted to do can't be done. But I managed to 
work around it in the end with a solution that was actually surprisingly 
simple.

For those interested to know what I was doing I was trying to implement 
a layer for prototype-based OO in Ruby. Didn't really work though. As 
far as I know (and please correct me if I'm wrong) Ruby doesn't allow 
you to call a method redefining the meaning of self for the purpose of 
the call (e.g. apply() in JS) and without this feature you can't really 
implement this layer in any useful way.
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