#include <boost/qvm/quat_operations.hpp>
namespace boost
{
    namespace qvm
    {
        //Only enabled if:
        //  is_quat<A>::value &&
        //  is_vec<B>::value && vec_traits<B>::dim==3
        template <class A>
        void set_rot( A & a, B const & axis, typename vec_traits<B>::scalar_type angle );
    }
}
As if:
assign( a, rot_quat(axis,angle));