On the documentation page: http://www.boost.org/doc/libs/1_42_0/libs/fusion/doc/html/fusion/functional.... Search for the sentence near the bottom of: The library provides both a strictly typed and a generic variant for this transformation. The latter should be used in combination with Boost.Functional/Forward to attack The Forwarding Problem. The words "Boost.Functional/Forward" are a link pointing to: http://www.boost.org/libs/functional/forward/doc/html/index.html However, that link does not exist, and I tried searching through: http://www.boost.org/libs/functional/ But I could not find anything that even had the word "forward" in it through any sub-link (courtesy of google, also searched in 1_41_0). Where is this documentation that Fusion is pointing to? Also, why does Phoenix2 not use the Fusion fused/unfused/etc... to support passing constants like the integer 42 directly to an actor? Is there some limitation that would not make it work, or...?