>Actually, it's not too hard to create a ReadOnlyString and ReadOnlyArray
>which have several methods overridden to produce errors.  Overriding at:
>and basicAt: goes an awefully long way.  Then, the Parser needs to be
>updated.  I did this once, but it was in an image too divergent from the
>main Squeak image that it could really be useful.  :|

Is it necessary to change the parser?

I had been thinking instead about changing the behavior of class 
String so that Strings are immutable (by redefining at:put:, 
replaceFrom:to:with:startingAt:, byteAt:put: and what else?), and 
creating a separate class MutableString for those few places where 
string mutation is really used.  I wonder how common this behavior is.

One tricky thing is that string mutation is sometimes used to create 
a string that will then be treated as immutable.  See, for example, 
Symbol asString.  This could (and probably should) be changed to use 
String withAll: aSymbol.  Mutating a string is OK while it is being 
constructed (how else could it be constructed?, but taboo once it has 
been handed off to the world.)

	Andrew