template<bool = true>
struct boost::hana::embedding< bool >
Marks a conversion between data types as being an embedding. 
To mark a conversion between two data types To and From as an embedding, simply use embedding<true> (or simply embedding<>) as a base class of the corresponding to_impl specialization. If a to_impl specialization does not inherit embedding<true> or embedding<>, then it is not considered an embedding by the is_embedded metafunction.
Tip
The boolean template parameter is useful for marking a conversion as an embedding only when some condition is satisfied. 
Example 
#include <vector>
namespace boost { 
namespace hana {
     template <typename To, typename From>
    struct to_impl<
std::vector<To>, std::vector<From>,
         when<is_convertible<From, To>::value>>
        : embedding<is_embedded<From, To>::value>
    {
        static std::vector<To> 
apply(std::vector<From> 
const& xs) {
             std::vector<To> result;
            for (auto const& x: xs)
                result.push_back(to<To>(x));
            return result;
        }
    };
}}
int main() {
        hana::to<std::vector<int>>(std::vector<float>{1.1, 2.2, 3.3})
                         ==
        std::vector<int>{1, 2, 3}
    );
    static_assert(!hana::is_embedded<std::vector<float>, std::vector<int>>{}, "");
}