Cannot set instance variables in NilClass

[masuidrive]

Cannot set instance variables in NilClass.

class NilClass
    def set_a
        @a = 0
    end
end
nil.set_a
=> ArgumentError: cannot set instance variable

[matz]

It's a limitation of mruby, unlike CRuby, immediate values cannot have instance variables.
If the real problem is instance variables at top-level, it will be addressed by #327.