Three corrections to the special missings post:
- a special missing is typed with or without its leading period - `.a` and `a`
are the same missing
- MINVAL and MAXVAL both accept a special missing, and both still reject a real
number that is out of range. A minimum constrains a number, and a missing is
not a number; NOTNULL is the rule for a column that must be populated
- the formula rules are the one case where a special missing genuinely does not
work, so the range rules are no longer grouped with them
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