gnu := blk.toGNU()
		sparse := gnu.sparse()
		copy(gnu.realSize(), size)
		sps = populateSparseMap(sparse, sps)
		if format != FormatUnknown {
			blk.setFormat(format)
		}
		out = append(out, blk[:]...)

		// Write extended sparse blocks.
		for len(sps) > 0 {
			var blk block
			sps = populateSparseMap(blk.toSparse(), sps)
			out = append(out, blk[:]...)
		}
		return out
	}