# File lib/image_formats/D64.rb, line 66
def get_sector(track_no,sector_no)
    return nil if @@sectors_in_track[track_no].nil?
                return nil unless sector_no.between?(0,@@sectors_in_track[track_no]-1)
                start_byte=@@offset_of_track[track_no]+(D64_BYTES_PER_SECTOR*(sector_no))    
                file_bytes[start_byte,D64_BYTES_PER_SECTOR]
end