Scott,
Evaluation of this form fails with an error:
(with-standard-io-syntax (let ((*readtable* (copy-readtable *readtable*)) (v (with-default (map) (set)))) (fset::fset-setup-rereading-readtable *readtable*) (with-input-from-string (s (with-output-to-string (s) (write (list v) :stream s))) (read s))))
..whereas this succeeds:
(with-standard-io-syntax (let ((*readtable* (copy-readtable *readtable*)) (v (map))) (fset::fset-setup-rereading-readtable *readtable*) (with-input-from-string (s (with-output-to-string (s) (write (list v) :stream s))) (read s))))
...which probably suggests that the reader hasn't been conditioned to take care of the trailing "/foo" syntax.