Well, Arc lists are (e e . nil). Why do we have a different terminator for strings?

This is where things get hairy. In Scheme lists are (e e . ()), so I would say that having strings be (#\a #\b . "") would make sense there, but Arc specifically tries to pretend that () is nil. Why should "" be different from nil, when () is nil?

As for [isa _ 'string], I propose instead the following function:

  (def astring (e)
    ((afn (e e2)
       (if
         (no e)
           t
         ; check for circularity
         (is e e2)
           nil
         (and (acons e) (isa (car e) 'character))
           (self (cdr e) (cddr e2))
         ; else
           nil))
      e (cdr e)))

Hmm. I suppose someone will complain that you might want to differentiate between the empty string and nil. To which I respond: How is nil different from the empty list? Arc attempts to unify them; why do we want to not unify the empty string with nil? If someone says http://example.com/foobar?x= , how different is that from http://example.com/foobar ? x is still empty/nil in both cases.