code: public /*@Nullable*/String createCopyRef(String path)
          throws DbxException
      {
          DbxPathV1.checkArgNonRoot("path", path);
  
          String apiPath = "1/copy_ref/auto" + path;
  
          return doPost(host.getApi(), apiPath, null, null, new DbxRequestUtil.ResponseHandler</*@Nullable*/String>()
          {
              @Override
              public /*@Nullable*/String handle(HttpRequestor.Response response)
                  throws DbxException
              {
                  if (response.getStatusCode() == 404) return null;
                  if (response.getStatusCode() != 200) throw DbxRequestUtil.unexpectedStatus(response);
                  CopyRef copyRef = DbxRequestUtil.readJsonFromResponse(CopyRef.Reader, response);
                  return copyRef.id;
              }
          });
      }
docs: Creates and returns a "copy ref" to a file.  A copy ref can be used to copy a file across
  different Dropbox accounts without downloading and re-uploading.
  
  <p>
  For example, create a {@code DbxClientV1} using the access token from one account and call
  {@code createCopyRef}.  Then, create a {@code DbxClientV1} using the access token for another
  account and call {@code copyFromCopyRef} using the copy ref.
  </p>
  
  <p>
  A copy ref created by an app can only be used by that app.
  </p>
  
  @param path
  The Dropbox path to a file.
  
  @return
  The copy ref's identifier, suitable for passing in to {@link #copyFromCopyRef},
  or {@code null} if the specified path does not exist.