- add enqueue-front!
- add queue-filter!
- use the predicates instead of the /c contracts
- make queue-length take constant time
- add some random tests
- note the running times of all of the operations in the docs
- make queues be sequences directly (and use make-do-sequence
to implement in-queue instead of building a list)
- added non-empty-queue? (note extra hypen as compared
to the past; this seems better since the function
wasn't exported before and we already have other
functions named "non-empty-<something>" but not
others namedn "nonempty-<something>")
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