datatype BoolList
	= TRUE of BoolList
	| FALSE of BoolList
	| NULL

fun getTrueCount 	(NULL) 	  	= 0
	| getTrueCount 	(TRUE(x))  	= 1 + getTrueCount x
	| getTrueCount 	(FALSE(x)) 	= getTrueCount x;

fun xor x = getTrueCount x mod 2;

fun remove (flag, NULL) = NULL
	| remove (flag, TRUE(x)) = if flag then remove(flag, x) 		else TRUE(remove(flag, x))
	| remove (flag, FALSE(x))= if flag then FALSE(remove(flag, x)) 	else remove(flag, x);


fun toString (TRUE(x)) 		= "TRUE " ^ toString(x)
	| toString (FALSE(x))	= "FALSE " ^ toString(x)
	| toString (NULL) 		= "";

fun processList (t, f, n) NULL = n(NULL)
	| processList (t, f, n) (TRUE(x)) = t(processList (t, f, n) x)
	| processList (t, f, n) (FALSE(x))= f(processList (t, f, n) x);

val getTrueCount1 = processList(fn x => x + 1, fn x => x , fn x => 0);
val xor1 = processList(fn x => (x+1) mod 2, fn x => 0, fn x => 0);
fun remove1 (flag) = 
	if flag then processList(fn x => x, fn x => FALSE(x), fn x => x)
	else processList(fn x => TRUE(x), fn x => x, fn x => x);
val toString1 = processList(fn x => "TRUE " ^ x, fn x => "FALSE " ^ x, fn x => "");