Table conversion

[Eric Dean Tribble <tribble>]

We'll need standard messages for making read-only views (Const views)
on tables and sets, making an immutable version of the state of a
Table or Set, etc.  

May I suggest asImmutable to create an immutable version of a table or
set (that's why the lowercase 'T').  The message 'asConst' could
create a read-only view on a table.  Note that a correct
implementation returns self for appropriate classes in the hierarchy.

dean