>I think that one can make a compelling argument that strings should 
>be completely immutable.

By all means make it.  It would be nice if someone would spell it 
out.  I can think of many advantages an implementation could use to 
take advantage of this,  (thin string slices, substrings, etc.), but 
unsurprinsingly there are costs as well.  Why not support both types?

Does the ANSI standard speak to this point at all?