// predicates: L<F1<S, Bool>>
// modifies & returns tok
static LS groupTokenPredicateSequence(LS tok, L predicates) {
  int min = 0;
  for (int i : indicesOfTokenPredicateSequence(tok, predicates)) {
    continue if i <= min;
    int j = l(predicates)*2-2;
    tok_addCurlyBracketsAroundPart(tok, i, j);
    min = j;
  }
  ret tok;
}