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The value field of the CHECKSUM keyword shall contain a 16 character string, left justified starting in column 12, containing the ASCII encoded complement of the checksum of the FITS HDU (Header and Data Unit). The algorithm shall be the 32-bit 1's complement checksum and the ASCII encoding that are described in §3. The checksum is accumulated in FITS datastream order on the same HDU, identical in all respects, except that the value of the CHECKSUM keyword shall be set to the string '0000000000000000' (ASCII 0's, hex 30) before the checksum is computed. Other equivalent interpretations of the ASCII encoding algorithm are possible, see §3.6.