}
				if len(sp) > 0 {
					sa.IsExtended()[0] = 1
				}
				return sp
			}
			sp2 := formatSPD(spd, blk.GNU().Sparse())
			for len(sp2) > 0 {
				var spHdr block
				sp2 = formatSPD(sp2, spHdr.Sparse())
				spb = append(spb, spHdr[:]...)
			}

			// Update size fields in the header block.
			realSize := hdr.Size
			hdr.Size = 0 // Encoded size; does not account for encoded sparse map