static ItIt<S> sentencePrefixesIterator(S s, O... _) {
  boolPar plusFull;
  LS tok = javaTokNPunctuationWithBrackets(s);
  ret mapI(countIterator_exclusive_step(3, l(tok)+toInt(plusFull), 2), i ->
    dropTrailingComma(trimJoinSubList(tok, 0, i)));
}