code: func (otp HOTP) Increment() {
  	for i := ctrSize - 1; i >= 0; i-- {
  		if otp.counter[i]++; otp.counter[i] != 0 {
  			return
  		}
  	}
  }
docs: // Increment will increment an HOTP source's counter. This is useful
  // for providers like the Google Authenticator app, which immediately
  // increments the counter and uses the 0 counter value as an integrity
  // check.