This is where "code is spec" breaks down, again ^^; \/
I suppose if the user uses symbol syntax, it's an immutable string, while if the user uses "string syntax", it's a mutable string. Interface, anyone? ^^
edit: typical lisps implement symbols as references to mutable strings; some newer ones implement mutable strings as references to immutable strings, with symbols referring also to immutable strings.
This isn't so much code is spec, though: Arc only has mutable strings and symbols. You could consider symbols immutable strings, but they exist to fill a different niche.[1] If mutable and immutable strings were created, then the code-spec would have to deal with this; I think it would be capable of doing so.
I'm not so concerned with how Lisps represent symbols and (mutable) strings as long as (1) my strings can be modified, and (2) comparing symbols takes constant time. If it's the Lisp interpreter protecting the string-representing-the-symbol, so be it; that doesn't affect me as a Lisp programmer.
[1]: Although if I recall, Ruby 2.0 will make its Symbols "frozen" (immutabilified) Strings, thus allowing things like regex matching on Symbols. This might be an interesting approach...
