Perhaps an underscore:

  class X
    def _foo
      puts "I'm local to X."
    end
  end

I suggest that b/c the idea of local instance vars in the form of @_a
has also been given consideration.

Though, I sort of like the idea of:

  def @foo
    ...
  end

But maybe that only makes sense if all instance vars are local and
accessors are required to communicate between class and subclass.

T.