code: func (d *driver) Writer(ctx context.Context, path string, append bool) (storagedriver.FileWriter, error) {
  	blobRef := d.client.GetContainerReference(d.container).GetBlobReference(path)
  	blobExists, err := blobRef.Exists()
  	if err != nil {
  		return nil, err
  	}
  	var size int64
  	if blobExists {
  		if append {
  			err = blobRef.GetProperties(nil)
  			if err != nil {
  				return nil, err
  			}
  			blobProperties := blobRef.Properties
  			size = blobProperties.ContentLength
  		} else {
  			err = blobRef.Delete(nil)
  			if err != nil {
  				return nil, err
  			}
  		}
  	} else {
  		if append {
  			return nil, storagedriver.PathNotFoundError{Path: path}
  		}
  		err = blobRef.PutAppendBlob(nil)
  		if err != nil {
  			return nil, err
  		}
  	}
  
  	return d.newWriter(path, size), nil
  }
docs: // Writer returns a FileWriter which will store the content written to it
  // at the location designated by "path" after the call to Commit.