[boost] [program_options] documentation for validators

Hi, I have a question regarding the validators namespace of program_options documentation. That namespace is referenced in the tutorial in http://www.boost.org/doc/libs/1_55_0/doc/html/program_options/howto.html and some of its functions are used but there is no documentation related with that namespace. Actually, it does not even appears in the reference manual of the lib. Is that intended ? Regards Alain