static <A> L<A> replace(L<A> l, A a, A b) {
  for i over l:
    if (eq(l.get(i), a))
      l.set(i, b);
  ret l;
}

static S replace(S s, S a, S b) {
  ret s == null ? null : a == null || b == null ? s : s.replace(a, b);
}