# File lib/image_formats/D64.rb, line 73
def set_sector(track_no,sector_no,sector_contents)
    raise "invalid track number #{track_no}" if @@sectors_in_track[track_no].nil?
    raise "invalid sector number #{sector_no}" unless sector_no.between?(0,@@sectors_in_track[track_no]-1)
    padded_sector_contents=(sector_contents+("\000"*D64_BYTES_PER_SECTOR))[0,D64_BYTES_PER_SECTOR]
                start_byte=@@offset_of_track[track_no]+(D64_BYTES_PER_SECTOR*(sector_no))    
                file_bytes[start_byte,D64_BYTES_PER_SECTOR]=padded_sector_contents
end