private RopeByteString(ByteString left,ByteString right){
  this.left=left;
  this.right=right;
  leftLength=left.size();
  totalLength=leftLength + right.size();
  treeDepth=Math.max(left.getTreeDepth(),right.getTreeDepth()) + 1;
}

private RopeByteString ( ByteString left , ByteString right ) { this . left = left ; this . right = right ; leftLength = left . size ( ) ; totalLength = leftLength + right . size ( ) ; treeDepth = Math . max ( left . getTreeDepth ( ) , right . getTreeDepth ( ) ) + 1 ; }
create a new ropebytestring , which can be thought of as a new tree node , by recording references to the two given strings .
