Problem Statement

Define the data structure for a doubly-linked list element. The element should include a data member to hold its value and pointers to both the next element in the list and the previous element in the list. The pointers should be mutable.

Let's start with the solution:

type 'a DLElm = {
    mutable prev: 'a DLElm option
    data: 'a
    mutable next: 'a DLElm option
}