code: func truncate(in []byte) int64 {
  	offset := int(in[len(in)-1] & 0xF)
  	p := in[offset : offset+4]
  	var binCode int32
  	binCode = int32((p[0] & 0x7f)) << 24
  	binCode += int32((p[1] & 0xff)) << 16
  	binCode += int32((p[2] & 0xff)) << 8
  	binCode += int32((p[3] & 0xff))
  	return int64(binCode) & 0x7FFFFFFF
  }
docs: // truncate contains the DT function from the RFC; this is used to
  // deterministically select a sequence of 4 bytes from the HMAC
  // counter hash.