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A queue is a first-in, first-out store of information. This implementation of queues uses difference-lists, the head of the difference-list represents the beginning of the queue and the tail represents the end of the queue. The members of the difference-list are the elements in the queue. The first argument in the queue-representation is the number of elements in the queue in unary representation.
Thus, a queue with n elements is represented as follows:
q(s(...s(0)...), [X1,...,Xn,Y1,...,Ym], [Y1,...,Ym])
where n is the length of the queue and X1...Xn are the elements of the queue.
To load the package, enter the query
| ?- use_module(library(queues)).
| ?- queue_head(Head, Nq, q(s(s(s(s(0)))),[1,2,3,4|R],R)). Head = 1, Nq = q(s(s(s(0))),[2,3,4|_193],_193), R = _193 ? yes
| ?- queue_last_list([5,6], q(s(s(0)))), [1,2|R], R), NQ). NQ = q(s(s(s(s(0)))))),[1,2,5,6|_360],_360), R = [5,6|_360] ? yes
| ?- list_queue([1,2,3,4], Q). Q = q(s(s(s(s(0)))),[1,2,3,4|_138],_138) ? yes | ?-
| ?- queue_length(q(s(s(s(s(s(0))))),[a,b,c,d,e|R],R), L). L = 5, R = _155 ? yes
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