When building on Mac, I'm getting several errors like so:

In file included from /usr/local/include/boost/multi_index/ordered_index.hpp:17:
/usr/local/include/boost/multi_index/detail/ord_index_impl.hpp:1172:25: error:
      no matching function for call to object of type 'const key_compare' (aka
      'const CompareTxMemPoolEntryByScore')
    return y==header()||comp_(key(v),key(y->value()));

The comp_(key(v)....  function is the reason for the error.