tables and Becomes

[Eric Dean Tribble <tribble>]

Markm made a proposal that I rather like if it can work.  Essentially,
we implement that NotAType modifier to classes, then objects can only
be converted to other NotAType classes that are subclasses of the same
type.  You can access protocol specific to NotAType classes by casting
to them when you send the message (you just can't declare a variable).