The EXTEND procedure opens an existing file, positions the  file
  buffer  after  the  last component, and prepares it for writing.
  It is commonly used to append to a file.

  Syntax:

     EXTEND( file_variable [[, file_name]]
             [[, ERROR := error-recovery]] );

  The 'file_variable' is the name of the file variable  associated
  with the output file.

  The 'file_name' is the name of the file to  be  associated  with
  the  'file_variable'.   If  the  file  was  opened with the OPEN
  procedure, the 'file_name' is ignored.

  The 'error-recovery' is the action  to  be  taken  if  an  error
  occurs during execution of the routine.

  The file can be in any mode before EXTEND is called to  set  the
  mode  to generation.  If the file is an external file and is not
  already open, EXTEND opens it using the defaults  for  the  OPEN
  procedure.

  After execution of EXTEND, the file is positioned after the last
  component,  and  EOF  and UFB return TRUE.  If the file does not
  exist, EXTEND does not create it, but returns an  error  at  run
  time.

  A call to EXTEND on a relative file  opened  for  direct  access
  positions the file after its last existing component.

  A call to EXTEND on an indexed file opened for random access  by
  key  positions the file after the last component relative to the
  primary key.

  See the "HP Pascal Language Reference  Manual"  for  a  complete
  description of the EXTEND procedure.