An iterative version of append can copy the first list in a loop, using O(1) stack space. For this function, it is convenient to use the setrest function:
public static Cons append (Cons x, Cons y) {
Cons front = null;
Cons back = null;
Cons cell;
if ( x == null ) return y;
for ( ; x != null ; x = rest(x) ) {
cell = cons(first(x), null);
if ( front == null )
front = cell;
else setrest(back, cell);
back = cell; }
setrest(back, y);
return front; }