The following are examples in SRFI-89:

    (define (h2 [key: k #f] a . r) (list a k r))

    (h2 7)                 ==>  (7 #f ())
    (h2 7 8 9 10)          ==>  (7 #f (8 9 10))
    (h2 key: 8 9 10)       ==>  (9 8 (10))
    (h2 key: 8 zoo: 9)     ==>  error

1. If we make the required positional variable `a' to take a keyword
   object as its value, is it possible?

2. I don't know what the `positional' of `required positional' means.
   (h2 [key: k #f] a . r) --> the second parameter
   (h2 7 8 9 10)	  --> the first actual argument
   (h2 key: 8 9 10)	  --> the third actual argument

--
Joo ChurlSoo