[Checkers] Type Declaration Qualifiers

[Michael Ernst]

Mahmood-

> >> How should one express that a type cannot be Immutable?
> > Can you give a use case for when one would want to say this?
> I am thinking of procedural classes, e.g. loggers, parsers, output  
> streams.  In practice, all references are mutable references; and I  
> cannot imagine having an immutable reference to such classes.

I see.  Declaring one Immutable will quickly cause a type error at the
useful operations, so I don't think people would persist in such an error
for very long.  Thus, although such a declaration might be nice to have, it
doesn't feel like to me like it is essential.  But let's keep it in mind
for the future.

                    -Mike