/** Prepend given list of elements to front of list, forming and returning a new list. */
public List<A> prependList(List<A> xs) {
if (this.isEmpty()) return xs;
if (xs.isEmpty()) return this;
if (xs.tail.isEmpty()) return prepend(xs.head);
// return this.prependList(xs.tail).prepend(xs.head);
List<A> result = this;
List<A> rev = xs.reverse();
Assert.check(rev != xs);
// since xs.reverse() returned a new list, we can reuse the
// individual List objects, instead of allocating new ones.
while (rev.nonEmpty()) {
List<A> h = rev;
rev = rev.tail;
h.setTail(result);
result = h;
}
return result;
}
