def [reader, writer] := makeQueue()
require(escape empty { reader.dequeue(empty); false } catch _ { true })
writer.enqueue(1)
writer.enqueue(2)
require(reader.dequeue(throw) == 1)
writer.enqueue(3)
require(reader.dequeue(throw) == 2)
require(reader.dequeue(throw) == 3)
require(escape empty { reader.dequeue(empty); false } catch _ { true })