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 4.7 Rules without Commands or Prerequisites
 ===========================================
 
 If a rule has no prerequisites or commands, and the target of the rule
 is a nonexistent file, then `make' imagines this target to have been
 updated whenever its rule is run.  This implies that all targets
 depending on this one will always have their commands run.
 
    An example will illustrate this:
 
      clean: FORCE
              rm $(objects)
      FORCE:
 
    Here the target `FORCE' satisfies the special conditions, so the
 target `clean' that depends on it is forced to run its commands.  There
 is nothing special about the name `FORCE', but that is one name
 commonly used this way.
 
    As you can see, using `FORCE' this way has the same results as using
 `.PHONY: clean'.
 
    Using `.PHONY' is more explicit and more efficient.  However, other
 versions of `make' do not support `.PHONY'; thus `FORCE' appears in
 many makefiles.   Phony Targets.
 

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