static LS tok_copyRawNTokens(LS tokA, LS tokB) {
  if (l(tokA)*2-1 != l(tokB)) ret tokB;
  new LS tokC;
  for i over tokB:
    tokC.add(odd(i) ? tokB.get(i) : tokA.get(i/2));
  ret tokC;
}