Basic Ideas --- Construction 


Example:  append

append(x,y) = case x of 
 	nil 	    -> y
 	Cons[x1,xs] -> Cons[x1,append(xs,y)]

Find an outermost function call 
↓

append(x,y) = case x of 
 	nil 	    -> y
 	Cons[x1,xs] -> Cons[x1,DUMMY]append(xs,y)