Special generic values

Hall, S. R.; Spadaccini, N.; Brown, I. D.; Bernstein, H. J.; Westbrook, J. D.; McMahon, B.

doi:10.1107/97809553602060000728

International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

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International Tables for Crystallography (2006). Vol. G. ch. 2.2, p. 34

Section 2.2.7.4.8. Special generic values

S. R. Hall,^a ^* N. Spadaccini,^c I. D. Brown,^d H. J. Bernstein,^e J. D. Westbrook^b and B. McMahon^f

2.2.7.4.8. Special generic values

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(21) The unquoted character literals ? (query mark) and . (full point) are special and are valid expressions for any data type.

(22) The value ? means that the actual value of a requested data item is unknown.

(23) The value . means that the actual value of a requested data item is inapplicable. This is most commonly used in a looped list where a data value is required for syntactic integrity.

References

International Tables for Crystallography (2006). Vol. G. ch. 2.2, p. 34