code: func EncodeInt(b []byte, v int64) []byte {
  	var data [8]byte
  	u := encodeIntToCmpUint(v)
  	binary.BigEndian.PutUint64(data[:], u)
  	return append(b, data[:]...)
  }
docs: // EncodeInt appends the encoded value to slice b and returns the appended slice.
  // EncodeInt guarantees that the encoded value is in ascending order for comparison.