static PairS splitAtFirstDoubleArrow(S s) {
  ret splitAtFirstDoubleArrow(javaTok(s));
}

static PairS splitAtFirstDoubleArrow(LS tok) {
  int i = indexOfSubList(tok, splitAtDoubleArrow_tok());
  ret i < 0 ? null : pair(trimJoin(subList(tok, 0, i)), trimJoin(subList(tok, i+3)));
}