def read(nbytes)
      raise IncompleteBufferException if @idx + nbytes > @buffy.bytesize
      ret = @buffy.slice(@idx, nbytes)
      @idx += nbytes
      ret
    end
def read(nbytes)
      raise IncompleteBufferException if @idx + nbytes > @buffy.bytesize
      ret = @buffy.slice(@idx, nbytes)
      @idx += nbytes
      ret
    end
Retrieve the next _nbytes_ number of bytes from the buffer.