Eric, sorry for the delay. I only now found some time to work on that issue: http://lists.boost.org/boost-users/2007/08/30120.php After analysing your post and investigation what's going on, I would like to write my opinion: changing xpressive_fwd.hpp or proto_fwd.will not help. I grepped where fusion includes are used and need to enclose these in some conditional statements: #ifdef BOOST_XPRESSIVE_FUSION_V2 # include ... //fusion 2 headers #else # include ... // fusion 1 headers #endif Actually this can be done using some kind of a macro function like: #include BOOST_XPRESSIVE_FUSION_ROOT(sequence/cons.hpp) ... The macro function will deliver the root dependent on the predefined macros like: 1. if !defined XPRESSIVE_FUSION_ROOT and use V2 => choose the root for V2 else => choose the root for V1 As I found out the following files need to be changed: linker.hpp alternate_matcher.hpp alt_compilers.hpp any.hpp cons.hpp arg_traits.hpp op_base.hpp proto_fwd.hpp If you give me your green light I will do it and submit a patch. With Kind Regards, Ovanes