template <class ArgType>
class Circulant : public Eigen::MatrixBase<Circulant<ArgType> >
{
public:
  Circulant(const ArgType& arg)
    : m_arg(arg)
  { 
    EIGEN_STATIC_ASSERT(ArgType::ColsAtCompileTime == 1,
                        YOU_TRIED_CALLING_A_VECTOR_METHOD_ON_A_MATRIX);
  }

  typedef typename Eigen::internal::ref_selector<Circulant>::type Nested; 

  typedef typename Eigen::internal::traits<Circulant>::Index Index;
  Index rows() const { return m_arg.rows(); }
  Index cols() const { return m_arg.rows(); }

  typedef typename Eigen::internal::ref_selector<ArgType>::type ArgTypeNested;
  ArgTypeNested m_arg;
};