fifo | pop # produces [null,[]]
fifo
| push(42) # enqueue
| push(43) # enqueue
| pop # dequeue
| .[0] # the value
# produces 43
fifo|push(1) as $q1
| fifo|push(2) as $q2
| [($q1|pop|.[0]), ($q2|pop|.[0])]
# produces: [1, 2]