doc proper-list? etc

[Kevin Ryde]

I've tried to clarify proper-list? and friends a bit, in particular to
note how they partition all objects.



 -- Scheme Procedure: proper-list? obj
     Return `#t' if OBJ is a proper list, or `#f' otherwise.  This is
     the same as the core `list?' (*note List Predicates::).

     A proper list is a list which ends with the empty list `()' in the
     usual way.  The empty list `()' itself is a proper list too.

          (proper-list? '())       => #t
          (proper-list? '(1 2 3))  => #t

 -- Scheme Procedure: circular-list? obj
     Return `#t' if OBJ is a circular list, or `#f' otherwise.

     A circular list is a list where at some point the `cdr' refers
     back to a previous pair in the list (either the start or some later
     point), so that following the `cdr's takes you around in a circle,
     with no end.

          (define x (list 1 2 3 4))
          (set-cdr! (last-pair x) (cddr x))
          x => (1 2 3 4 3 4 3 4 ...)
          (circular-list? x)  => #t

 -- Scheme Procedure: dotted-list? obj
     Return `#t' if OBJ is a dotted list, or `#f' otherwise.

     A dotted list is a list where the `cdr' of the last pair is not
     the empty list `()'.  Any non-pair OBJ is also considered a dotted
     list, one with length zero.

          (dotted-list? '(1 2 . 3))  => #t
          (dotted-list? 99)          => #t

   It will be noted that any Scheme object passes exactly one of the
above three tests `proper-list?', `circular-list?' and `dotted-list?'.
Non-lists are `dotted-list?', finite lists are either `proper-list?' or
`dotted-list?', and infinite lists are `circular-list?'.