The UFB (undefined file buffer) function returns a Boolean value
  that  indicates  whether  the  last file operation gave the file
  buffer an undefined status.

  Syntax:

     UFB( file_variable )

  The 'file_variable' is the name of the file variable  associated
  with the file whose buffer is being tested.

  The file can be in any mode before UFB is called;  execution  of
  UFB does not change the file mode.

  UFB tests the effect of the last I/O operation done to the file.
  UFB  returns  FALSE  if a successful GET, FIND, FINDK, RESET, or
  RESETK operation has filled the file buffer.  GET, FIND,  FINDK,
  RESET,  and  RESETK  procedure  calls  that do not fill the file
  buffer set UFB to TRUE.

  UFB  also  returns  TRUE  after  DELETE,  EXTEND,  LOCATE,  PUT,
  REWRITE,  TRUNCATE, and UPDATE procedures have left the contents
  of the file buffer unknown.

  Assigning a new value to the  file  buffer  with  an  assignment
  statement does not change the value of UFB.

  See the "HP Pascal Language Reference  Manual"  for  a  complete
  description of the UFB function.