The parallel function takes as its arguments a set, an operator, and possibly additional arguments. It applies the operator to the set in parallel, and returns the empty set. Parallel may be called for its side effects, or as a means of manipulating the set elements. This implementation of parallel is actually sequential, but it is an error to use parallel in a way which relies on a particulat order of evaluation.
(define parallel
(function sset
((a sset) (b procedure) . c )
(letrec
((internal-ordered
(lambda (vector index length)
(if (< index length)
(begin (apply
b
(cons
a
(cons (vector-ref vector index)
(cons index c))))
(internal-ordered
vector (+ 1 index) length))
'())))
(internal-not
(lambda (elements operation)
(if (null? elements)
'()
(begin (apply operation
(cons a (cons (car elements)
(cons -1 c))))
(internal-not (cdr elements) operation))))))
(if (ordered? a)
(internal-ordered
(cadr a) 0 (vector-length (cadr a)))
(internal-not (cdr a) b))
'(set))))