29  auto& front() { 
return m_ref.front(); }
 
   30  const auto& front()
 const { 
return m_ref.front(); }
 
   32  auto empty()
 const { 
return m_ref.empty(); }
 
   34  auto size()
 const { 
return m_ref.size(); }
 
   35  auto begin()
 const { 
return m_ref.cbegin(); }
 
   36  auto end()
 const { 
return m_ref.cend(); }
 
   37  auto cbegin()
 const { 
return m_ref.cbegin(); }
 
   38  auto cend()
 const { 
return m_ref.cend(); }
 
   39  auto begin() { 
return m_ref.begin(); }
 
   40  auto end() { 
return m_ref.end(); }
 
   41  auto cbegin() { 
return m_ref.cbegin(); }
 
   42  auto cend() { 
return m_ref.cend(); }
 
   44  auto& operator[](std::size_t i) { 
return m_ref[i]; }
 
   45  const auto& operator[](std::size_t i)
 const { 
return m_ref[i]; }