panic("invalid format")
	}

	// Update checksum.
	// This field is special in that it is terminated by a NULL then space.
	var f formatter
	field := b.toV7().chksum()
	chksum, _ := b.computeChecksum() // Possible values are 256..128776
	f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
	field[7] = ' '
}

// computeChecksum computes the checksum for the header block.

	SHA1H V17, V25                                              // 390a285e
	SHA1M V0.S4, V27, V27                                       // 7b23005e
	SHA1P V3.S4, V20, V27                                       // 9b12035e
	SHA1SU0 V17.S4, V13.S4, V16.S4                              // b031115e
	SHA1SU1 V24.S4, V23.S4                                      // 171b285e

	AESE	V1.D2, V2.D2                                     // ERROR "invalid arrangement"
	AESIMC	V1.S4, V2.S4                                     // ERROR "invalid arrangement"
	SHA1SU1	V1.B16, V2.B16                                   // ERROR "invalid arrangement"
	SHA256SU1	V1.B16, V2.B16, V3.B16                   // ERROR "invalid arrangement"