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The "once" pragma can be used to indicate that the file which contains this
pragma should only be opened and processed "once".  The following describes
the form of the "once" pragma.

+--------------------------------------------------------------------------+
|      #pragma once [;]                                                    |
|                                                                          |
+--------------------------------------------------------------------------+
Assume that the file "foo.h" contains the following text.

Example:

     #ifndef _FOO_H_INCLUDED
     #define _FOO_H_INCLUDED
     #pragma once
         .
         .
         .
     #endif

The first time that the compiler processes "foo.h" and encounters the "once"
pragma, it records the file's name.  Subsequently, whenever the compiler
encounters a #include statement that refers to "foo.h", it will not open the
include file again.  This can help speed up processing of #include files and
reduce the time required to compile an application.