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2.4.11 The NOP Instruction
     NOP ;

The NOP instruction is the “no operation” statement; it does
nothing. Actually, that is not totally true, since the NOP
instruction is a “real” statement (and a placeholder), as opposed
to null clauses. I’ve only seen this used in two circumstances.

·    After any THEN or ELSE keyword, where a statement is
  required, when the programmer wants an empty THEN or ELSE part. By
  the way, this is the intended use of NOP.  Note that you can not
  use a null clause there (label, comment, or empty lines), since
  these are not parsed as “independent” statements.

·    I have seen it used as “trace-bait”. That is, when you start
  interactive trace, the statement immediately after the TRACE
  instruction will be executed before you receive interactive
  control.  If you don’t want that to happen (or maybe the TRACE
  instruction was the last in the program), you need to add an extra
  dummy statement.  However, in this context, labels and comments
  can be used, too.