(c) because in C an Fixnum is only an bitshift ...
    other ruby objects are struct RObject objects ... in this RObject 
there are flags and options and entries for instance variables and 
singleton methods


so there is no space where this stuff could be defined in a Fixnum

(in 1.9 it was possible to define instance variables into Fixnums but 
this was dropped in 2.0)

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