DCL_Command_Restrictions

   The PIPE command creates a special execution context for its
   command sequences. The following DCL commands either do not work
   or exhibit new behavior in this context:

   o  PIPE - Nested PIPE commands in the same command procedure
      level are not allowed. There can only be one PIPE command
      context for each command procedure level; however, nested
      PIPE commands at different procedure levels are allowed. For
      example:

       $ TYPE FOO.COM
       $ ! FOO.COM
       $   :
       $ PIPE   ...
       $ :
       $
       $ PIPE    @FOO.COM ; ...

      In this example, the PIPE command inside FOO.COM is allowed
      because it is executed at a different command procedure level.

   o  GOTO and EXIT - These two commands, when executed as PIPE
      command sequences, delete the PIPE command context before
      the GOTO or EXIT command is executed. Any command sequences
      following these two commands in a PIPE command are flushed.

   o  STOP - The STOP command, when executed after a PIPE command is
      interrupted by Ctrl/Y, deletes the PIPE command context.

   o  THEN, ELSE, ENDIF, SUBROUTINE, ENDSUBROUTINE, RETURN, and
      DCL labels - These commands cannot execute as PIPE command
      sequences because it is not possible to realize their
      functions in a PIPE command context.