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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 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_vector_buffer
          or basic_string member function that modifies the input sequence
          or output sequence.