transforming non tail recursive functions in tail recursive ones

	(* non tail recursive *)
	fun sum xs =
	case xs of
	[] => 0
	| x::xs' => x + sum xs'
	fun sumtailrecursive xs =
	let fun aux(xs,acc) =
	case xs of
	[] => acc
	| x::xs' => aux(xs', x + acc)
	in
	aux(xs,0)
	end
	(* whenever recursive operations are commutative we can use this approach
	of defining an auxiliary function which takes an extra accumulator parameter
	e.g. for summing or multiplying because
	3 * 4 == 4 * 3
	3 + 5 == 5 + 3
	*)
	(* non tail recursive *)
	fun reverse xs =
	case xs of
	[] => []
	| x::xs' => (reverse xs') @ [x]
	(* tail recursive *)
	fun reverse xs =
	let fun aux(xs, acc) =
	case xs of
	[] => acc
	| x::xs' => aux(xs', x::acc)
	in
	aux(xs,[])
	end

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