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DynamicBuffer_v1: 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
          bytes, resizing the basic_string object as necessary.
        
          An object of type mutable_buffers_type
          that satisfies MutableBufferSequence requirements, representing basic_string
          memory at the start of the output sequence of size n.
        
                If size()
                + n
                > max_size().
              
          The returned object is invalidated by any dynamic_string_buffer
          or basic_string member
          function that modifies the input sequence or output sequence.