Two-list functional queue

Represent the abstract sequence by front @ rev rear. Keep the front nonempty unless both lists are empty. Prepending to the rear enqueues; when the front is exhausted, reverse the rear once. Along a single sequence of uses, each element crosses between lists at most once, giving constant amortized analysis cost per operation. Reusing an old persistent version can repeat a reversal and needs a separate analysis.

// Destination: computer-science.bigb