fileid_value

 FILEID_VALUE - return the value of an FID whether it is an FID or inside a FILEOBJ

  FID = FILEID_VALUE(FID_OR_FILEOBJ)

  Given a value which may be a FID or a FILEOBJ object, return either the FID or
  the FID field of the FILEOBJ object.

  Allows one to test 'if fileid_value(f)<0' without knowing if f is a Matlab file identifier
  or a FILEOBJ.