mutable_buffers_type prepare( std::size_t n);
Ensures that the output sequence can accommodate
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
size() + n > max_size().
The returned object is invalidated by any
member function that modifies the input sequence or output sequence.