* It is typical to create a queue with empty initial state, and
  I think many existing implementations (e.g SLIB) has zero-argument
  make-queue to do so.  Is there a reason that not to make `k'
  optional with the default value 0?

* What will happen if I apply queue-front and queue-back on an empty
  queue?

* It hit me weird that it has queue-invoke specifically; is it such
  a common operation to deserve a separate API?  It's one-liner anyway,
  and I do use queues for such purpose, it's just one of many ways
  to use the queue.