// pred: char -> bool
sS takeCharsWhile(S s, O pred) {
  int i = 0;
  while (i < l(s) && isTrue(callF(pred, s.charAt(i)))) ++i;
  ret substring(s, 0, i);
}

sS lambdaMapLike takeCharsWhile(IPred<Char> f, S s) {
  ret takeCharsWhile(s, f);
}