Reasoning about Lists via List Interleaving

Among the various mathematical tools introduced in his outstanding work on Communicating Sequential Processes, Hoare has defined "interleaves" as the predicate satisfied by any three lists such that the first list may be split into sublists alternately extracted from the other two ones, whatever is the criterion for extracting an item from either one list or the other in each step.

This paper enriches Hoare's definition by identifying such criterion with the truth value of a predicate taking as inputs the head and the tail of the first list. This enhanced "interleaves" predicate turns out to permit the proof of equalities between lists without the need of an induction. Some rules that allow to infer "interleaves" statements without induction, particularly applying to the addition or removal of a prefix to the input lists, are also proven. Finally, a stronger version of the predicate, named "Interleaves", is shown to fulfil further rules applying to the addition or removal of a suffix to the input lists.