Basic Ideas --- Accumulation


append(x,y,<acc,num>) = case x of 
 	nil 	    -> accy
 	Cons[x1,xs] -> accCons[x1,DUMMY]
 		 	append(xs,y,<DUMMY,0>)

green part goes into <DUMMY,0> of append

↓

 	<DUMMYaccCons[x1,DUMMY], 2>
 ⇒	<accCons[x1,DUMMY], 2>