Delay of BOOST_PP_VARIADIC_ELEM fixes problem when variadic data must be generated.

svn path=/trunk/boost/preprocessor/; revision=73776
Update link to the C++ Template Metaprogramming book Appendix A online.

svn path=/trunk/libs/preprocessor/; revision=75125
Update link to the C++ Template Metaprogramming book Appendix A online.

svn path=/trunk/libs/preprocessor/; revision=75126
added VARIADIC_SEQ_TO_SEQ

svn path=/trunk/boost/preprocessor/; revision=80704