static BigInteger mathOp(S op, BigInteger a, BigInteger b) {
  if (eq(op, "+"))
    ret a.add(b);
  else if (eq(op, "-"))
    ret a.subtract(b);
  else if (eq(op, "*"))
    ret a.multiply(b);
  else if (eq(op, "/"))
    ret a.divide(b);
  else if (eq(op, "^"))
    ret a.pow(b.intValue()); // weird that it takes an int
  else
    throw fail("Unknown math op: " + op);
}