(define (add-node graph label . cluster-name)
(let* ((ns (nodes graph))
(id (length ns)))
(set-nodes! graph (assv-set! ns id label))
(if (and (not (null? cluster-name))
(define (add-node graph label . cluster-name)
(let* ((ns (nodes graph))
(id (length ns)))
(set-nodes! graph (assv-set! ns id label))
(if (and (not (null? cluster-name))