mutable_buffers_type prepare( std::size_t n);
Ensures that the output sequence can accommodate
resizing the vector object as necessary.
An object of type
mutable_buffers_type that satisfies MutableBufferSequence
requirements, representing vector memory at the start of the output sequence
size() + n > max_size().
The returned object is invalidated by any
vector member function that modifies the input sequence
or output sequence.