[Boost-users] what is template<typename> struct +; ?

While trying to read \boost\fusion adapt_struct.hpp and understand the internals of BOOST_FUSION_ADAPT_STRUCT. I came across this code: namespace boost { namespace mpl \ { \ template<typename> \ struct +; \ \ //.... }} Can anyone please explain what " template<typename> struct +; " means here? I am puzzled how it ever compiles as an operator is used as a class name. Thanks.