comp_char_type operator[](char_type c) const
 {
    if ( c >= m_strat->m_char.size() or !m_strat->m_char[c] )
         return (comp_char_type)0;
     return (comp_char_type) m_strat->m_char_rank((size_type)c);
 }
 char_type operator[](comp_char_type c) const
 {
     return (char_type) m_strat->m_char_select(((size_type)c)+1);
 }
 comp_char_type operator[](char_type c) const {
     return (comp_char_type) m_strat->m_char_rank((size_type)c);
 }