svoid tok_expandPairL(L<S> tok) {
  if (!tok.contains("PairL")) ret;
  int n = l(tok)-6;
  for (int i = 1; i < n; i += 2) {
    continue unless eq(tok.get(i), "PairL") && eq(tok.get(i+2), "<");
    int j = findEndOfTypeArgs(tok, i+2)-1; // j = index of >
    tok.set(i, "Pair<L");
    tok.set(j, ">>");
    reTok(tok, i, j+1);
    n = l(tok)-6;
  }
}

Began life as a copy of #1017528