mutable-red-black-node-color: contract violation

[sorawee]

Running https://github.com/sorawee/pareto-frontier/blob/master/test.rkt results in:

mutable-red-black-node-color: contract violation
  expected: mutable-red-black-node?
  given: #f

EDITED: making the program more minimal

#lang racket

(require rebellion/base/comparator
         rebellion/collection/sorted-set)

(define *set* (make-mutable-sorted-set #:comparator real<=>))

(sorted-set-add! *set* 49)
(sorted-set-add! *set* 68)
(sorted-set-add! *set* 80)
(sorted-set-add! *set* 53)
(sorted-set-add! *set* 27)
(sorted-set-add! *set* 34)
(sorted-set-add! *set* 48)
(sorted-set-remove! *set* 68)
(sorted-set-remove! *set* 80)

[jackfirth]

Closed by #508.