I went to look up the API on "flat_set" and found that the header documentation after 1.53 is not helpful. I think this is unintentional, so I thought I'd point it out: http://www.boost.org/doc/libs/1_53_0/doc/html/boost_container_header_referen... 1.53 docs: "class flat_set" is shown in the <flat_set.hpp> section; the link takes you to the API reference -- perfect! http://www.boost.org/doc/libs/1_54_0/doc/html/boost_container_header_referen... 1.54 docs: "class flat_set" only appears in <container_fwd.hpp> section; the link takes you to the forward decl only and not a reference header. However, other references to "flat_set" (for example, in the free functions like operator==) are hyperlinked and do take you to the right header reference file.. http://www.boost.org/doc/libs/1_55_0/doc/html/boost_container_header_referen... 1.55 docs: "class flat_set" is not listed at all. The API header reference does not exist and none of the references to flat_set are hyperlinked. Jared