the field 'previous' is called 'prev' in the rest of the implementation, so you probably want to rename it to 'prev' here...
This comment references this from milestone alpha3: http://www.realworldocaml.org/alpha3/en/html/imperative-programming-1.html#idp8451984
Now let's look at the implementation. We'll start by defining
'a element and 'a t.