A solution to the multiple inclusion problem

[PCSD Mac]

How about a new preprocessor directive that means "include this file only 
if it hasn't already been included", say "#require"?  The "preprocessor"
would simply keep a table of files that have already been included and use 
it to avoid including the same #required file more than once.  There will 
always be files you really do want to #include multiple times, so you can't
change the meaning of #include.

I believe that would do the trick, and it wouldn't break any existing 
programs.  If you use this construct though, you can't go back to compilers 
that don't support it.