t instantiated in any tuple.

TJ: Or:  a column might be defined as nullable, although
    all of the current rows have a value in that column.

TJ: In short:  that intensional rules might be inferrable from the
    full Cartesian Product of a set of columns (if we knew it was
    the full Cartesian Product), but are not inferrable from any
    subset of the full Cartesian Product.

o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o~~~~~~~~~o

</PRE>

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