An input stream.
A stream buffer.
Bounding indexes for a subsequence of buffer .
The generic function
is called by stream-fill-buffer to place characters into the region of the buffer
stream should be an instance of a subclass of buffered-stream.
should block until some data is available.
should be the number of characters actually placed in the buffer (0 if at end of file). This method must be implemented for subclasses of buffered-stream that handle input.