string-set! examples in r5rs.html

[Bill Schottstaedt]

according to r5rs.html, these should signal an error, I believe:

guile> (string-set! (symbol->string 'immutable)
             0
             #\?)
guile> (define (g) "***")
guile> (string-set! (g) 0 #\?)
guile> (g)
"?**"