code: func randomUint64(r io.Reader) (uint64, error) {
  	rv, err := binarySerializer.Uint64(r, bigEndian)
  	if err != nil {
  		return 0, err
  	}
  	return rv, nil
  }
docs: // randomUint64 returns a cryptographically random uint64 value.  This
  // unexported version takes a reader primarily to ensure the error paths
  // can be properly tested by passing a fake reader in the tests.