I wanted to retain annotate type information, so I decided to use a length-4 vector (annotate uses a length 3 vector). Also, by adding a hash in the 4th entry, we can attach even more keys to an object. For example, it would be possible to attach another function that returns the virtual "length" of the object, another function which returns the virtual "keys" of the object, and overload the basic functions (len ...) and (maptable ...) to use them.
