return nil
	}
	for part, partIdx := range partNums {
		if len(files[partIdx]) == 0 {
			continue
		}
		var wantMD5 string
		exportedSizes := make(map[int]bool)
		// block -> data
		combineSharedBlocks := make(map[int][]byte)
		combineFilledBlocks := make(map[int][]byte)
	nextFile:
		for key, file := range files[partIdx] {
			fmt.Println("Reading base version", file[0], "part", part)
			var combined []byte