static PositionalTokenIndex test_PositionalTokenIndex() {
  new PositionalTokenIndex idx;
  idx.addAll(map wordTok(tlft([[
    you are here
    what is this
    hello world
    hello you
  ]])));
  printStruct(idx);
  assertEqualsVerbose(ll("you are here"), map join(idx.byToken(0, "you")));
  // index should preserve original order
  assertEqualsVerbose(ll("hello world", "hello you"), map join(idx.byToken(0, "hello")));
  assertEqualsVerbose(ll("what is this"), map join(idx.byToken(1, "is")));
  assertEqualsVerbose(ll(), unnull(map join(idx.byToken(3, "is"))));
  stresstest_PositionalTokenIndex();
  ret idx;
}