[boost] [preprocessor]undef file iteration include guard?

The test driver in http://boost-consulting.com/vault /Preprocessor Metaprogramming/ctor_template.zip only works if the ctor_template.hpp file #undef's the include guard as follows: #undef boost_forwarder_ctor_template_included However, this need is not mentioned in: http://www.boost.org/libs/preprocessor/doc/topics/file_iteration.html Maybe it should be mentioned there or am I missing something?