13.1.1 SUB Subprograms
  A SUB subprogram is a program module that can be sep-
  arately compiled and that cannot return a value. A SUB
  subprogram is delimited by the SUB and END SUB state-
  ments. You may use the EXTERNAL statement to explicitly
  declare the SUB subprogram.

  The END SUB statement does the following:

  .
        Marks the end of the SUB subprogram
  .
        Does not affect I/O operations or files
  .
        Releases the storage allocated to local variables
  .
        Returns control to the calling program

  The EXIT SUB statement transfers control to the statement
  lexically following the statement that invoked the subpro-
  gram. It is equivalent to an unconditional branch to an END
  SUB statement.

  The following SUB subprogram sorts two integers. If this
  SUB statement is invoked with actual parameter values
  that are already in sorted order, the EXIT SUB statement is
  executed and control returns to the calling program.
  SUB sort_out (INTEGER X, INTEGER Y)
  DECLARE INTEGER temp
      IF X > Y
      THEN
            temp = X
            X = Y
            Y = temp
      ELSE
            EXIT SUB
      END IF
  END SUB