static BigInteger mul(BigInteger a, BigInteger b) {
  ret a.multiply(b);
}

static BigInteger mul(BigInteger a, long b) {
  ret a.multiply(bigint(b));
}

static int mul(int a, int b) { ret a*b; }
static long mul(long a, long b) { ret a*b; }
static double mul(double a, double b) { ret a*b; }