#include <boost/qvm/vec_operations.hpp>
namespace boost
{
    namespace qvm
    {
        //Only enabled if:
        //  is_vec<R>::value && is_vec<A>::value &&
        //  vec_traits<R>::dim==vec_traits<A>::dim
        template <class R,class A>
        R convert_to( A const & a );
    }
}
R must be copyable.
As if: R r; assign(r,a); return r;