In the SRFI document, it states that the lambda-list of
'delete-duplicates!'
should be something like:

(lis . maybe-=)

In the distribution of an implementation of SRFI 1 that comes with
Scheme48/scsh, the lambda-list is indeed:

(lis . maybe-=)

But in the reference implementation, it has a lambda-list of:

(lis maybe-=)

being defined as:

(define (delete-duplicates! lis maybe-=) ...)