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Get a list of buffers that represents the output sequence, with the given size.
mutable_buffers_type prepare( std::size_t n);
          Ensures that the output sequence can accommodate n
          characters, reallocating character array objects as necessary.
        
          An object of type mutable_buffers_type
          that satisfies MutableBufferSequence requirements, representing character
          array objects at the start of the output sequence such that the sum of
          the buffer sizes is n.
        
                If size()
                + n
                > max_size().
              
          The returned object is invalidated by any basic_streambuf
          member function that modifies the input sequence or output sequence.