// mapLL withDollar("1.23", "+1.23", "-0.12", "abc", null, "", "$1")
sS withDollar(S s) {
  if (empty(s)) ret "";
  if (startsWithOneOf(s, "$", "+$", "-$")) ret s;
  
  int i = 0;
  if (startsWithOneOf(s, "+", "-")) ++i;
  ret insertStringAt(s, i, "$");
}