static void sub(long[] output, long[] in1, long[] in2) {
    for (int i = 0; i < LIMB_CNT; i++) {
      output[i] = in1[i] - in2[i];
    }
  }
Find the difference of two numbers: output = in1 - in2 (note the order of the arguments!).

<p>On entry: in1, in2 are in reduced-size form.
java_src/src/main/java/com/google/crypto/tink/internal/Field25519.java