This macro expands by default to 
64. For a C++
      standard conforming preprocessor, the end-user can change this to
      128 or 256 by defining this macro to either value before including
      any library headers. If the end-user changes the value of the
      macro the 
BOOST_PP_LIMIT_VARIADIC
      macro is changed to the same value unless the end-user has changed
      BOOST_PP_LIMIT_VARIADIC to a higher value.