The EXTERNAL  statement  declares  constants,  variables,  functions,  and
  subroutines  external  to  your  program.  You can describe parameters for
  external functions and subroutines.

  Examples:

  EXTERNAL INTEGER CONSTANT SS$_NORMAL
  EXTERNAL INTEGER Event_flag
  EXTERNAL INTEGER FUNCTION Rect_area (REAL BY REF, REAL BY REF)
  EXTERNAL SUB File_open (LONG BY VALUE, STRING)