val test_array = ["123","12345","1234567","987654321","10001","~10001","~123","~100","100","~12345","1","2","~1","~10","2002","~2002","0"];

	fun even (x rem 2 = 0) = true | _ = false;

	fun middleThreeDigits
			(h :: t, s, 1 ) = s @ " --> too small"
		|	(h :: t, s, 2 ) = s @ " --> has even digits"
		|	(h :: t, s, 3 ) where (len (h :: t) = 3) = s @ " --> " @ (implode (h :: t))
		|	(h :: t, s, 3 ) = (middleThreeDigits ( sub (t, 0, (len t)-1), s, 3))
		|	(h :: t, s, m) = if len (h :: t) < 3 then
							middleThreeDigits (h :: t, s, 1)
						  else
						    if even ` len (h :: t) then
							  middleThreeDigits (h :: t, s, 2)
							else
							  middleThreeDigits (h :: t, s, 3)


		|	s	= if sub (s, 0, 1) = "~" then
				middleThreeDigits (sub (explode s, 1, len s), s, 0)
			else
				middleThreeDigits (explode s, s, 0)
	;

	map (println o middleThreeDigits) test_array;