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.
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