Extended List module

[wojpok]

This code is part of a pretty-printer PR #256 . This code have been separated from main PR for easier review and merging.

[ppolesiuk]

The invariant that dropLastAux works on non-empty list can be easily ensured by passing the head and the tail of the list separately. In my opinion such an implementation is more clear and doesn't use suspicious impossible function. Same comment applies to other last-related functions, as well as foldRight1* functions.

Suggested change

	let rec dropLastAux xs =
	match xs with
	| [] => impossible ()
	| [x] => []
	| x :: xs => x :: dropLastAux xs
	end in
	if isEmpty xs then
	None
	else
	Some (dropLastAux xs)
	let rec dropLastAux y xs =
	match xs with
	| [] => []
	| x :: xs => y :: dropLastAux x xs
	end in
	match xs with
	| [] => None
	| x :: xs => Some (dropLastAux x xs)
	end

[wojpok]

Cool trick. I will apply that