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4.1 Rule Example
================

Here is an example of a rule:

     foo.o : foo.c defs.h       # module for twiddling the frobs
             cc -c -g foo.c

   Its target is 'foo.o' and its prerequisites are 'foo.c' and 'defs.h'.
It has one command in the recipe: 'cc -c -g foo.c'.  The recipe starts
with a tab to identify it as a recipe.

   This rule says two things:

   * How to decide whether 'foo.o' is out of date: it is out of date if
     it does not exist, or if either 'foo.c' or 'defs.h' is more recent
     than it.

   * How to update the file 'foo.o': by running 'cc' as stated.  The
     recipe does not explicitly mention 'defs.h', but we presume that
     'foo.c' includes it, and that is why 'defs.h' was added to the
     prerequisites.