[boost] [Proto] Expression as fusion/MPL sequence

Seems somethign crooky on this front. Calling fusion::at_c on expression ends up in error even after including boost/proto/fusion.hpp. Same way, flatten used as a transform seems to not give me a type that can be passed to any fusion or mpl function. Looking at proto/fusion.hpp I noticed that the iterator is indeed random_access but not the view itself which as a forward_traversal tag. Even after fixing this, no dice, at_c<N>(some_proto_expr) still fails to compile.