
head_queue(Queue, Head)

   unifies Head with the first element of the queue

Type
   library(queues)

Description

    unifies Head with the first element of the queue.  The tricky part
    is that we might be at the end of a queue: Back-Back, with Back a
    variable, and in that case this predicate should not succeed, as we
    don't know what that element is or whether it exists yet.
    


