[lwip-users] why is not the file "lwipopts.h" in the lwip package?

[roboy]

I have downloaded a lwip 1.3.0 package, there is a file named opt.h. In opt.h, 
there is one line : 

#include "lwipopts.h"

but I can not find lwipopts.h in the downloaded package. 

Later, I have found the file in a "contrib" package.

So, what is the function of lwipopts.h? and why isn't it in a standard lwip 
1.3.0 package?

Thanks a lot.
                                
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roboy
2009-02-19