>But in a general Ruby programming view, where one programmer
>is controlling all the files, is it good practice for a superclass to
>use a method (or instance variable) defined by a derived class?
>This does not seem to follow from the library designer view.

I would probably put a placeholder in the superclass just to remind myself
that the method needs to be implemented in all subclasses.

	class Shape

		def area
			raise "Shape#area not defined in subclass"
		end
	
	end

-- Niklas