title: Unordered (hash) map from bitset to bitset on boost
body: <p>I want to use a cache, implemented by boost's <code>unordered_map</code>, from a <code>dynamic_bitset</code> to a <code>dynamic_bitset</code>. The problem, of course, is that there is no default hash function from the bitset. It doesn't seem to be like a conceptual problem, but I don't know how to work out the technicalities. How should I do that?</p>