code: BlockId append(String fileName, IoBuffer buffer) {
  		try {
  			IoChannel fileChannel = getFileChannel(fileName);
  
  			// Rewind the buffer for writing
  			buffer.rewind();
  
  			// Append the block to the file
  			long newSize = fileChannel.append(buffer);
  
  			// Return the new block id
  			return new BlockId(fileName, newSize / BLOCK_SIZE - 1);
  
  		} catch (IOException e) {
  			e.printStackTrace();
  			return null;
  		}
  	}
docs: Appends the contents of a byte buffer to the end of the specified file.
  
  @param fileName
  the name of the file
  @param buffer
  the byte buffer
  @return a block ID refers to the newly-created block.