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Rules for Cleaning the Directory

   Compiling a program is not the only thing you might want to write
rules for.  Makefiles commonly tell how to do a few other things besides
compiling a program: for example, how to delete all the object files
and executables so that the directory is `clean'.

   Here is how we could write a `make' rule for cleaning our example

             rm edit $(objects)

   In practice, we might want to write the rule in a somewhat more
complicated manner to handle unanticipated situations.  We would do

     .PHONY : clean
     clean :
             -rm edit $(objects)

This prevents `make' from getting confused by an actual file called
`clean' and causes it to continue in spite of errors from `rm'.  (See
Note: Phony Targets, and Note: Errors in Commands.)

A rule such as this should not be placed at the beginning of the
makefile, because we do not want it to run by default!  Thus, in the
example makefile, we want the rule for `edit', which recompiles the
editor, to remain the default goal.

   Since `clean' is not a dependency of `edit', this rule will not run
at all if we give the command `make' with no arguments.  In order to
make the rule run, we have to type `make clean'.  Note: How to Run

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