Logical Implication

Hi.

I would like to intruduce an IMPLIES operator for booleans that reads
better than its developed formula.  That is, I think

    a IMPLIES b

is better in CHECK constraints and elsewhere than

    NOT a OR b

.

I plan on submitting this to the SQL committee as well, but I've learned
that they like an implementation to have it first, and I cannot imagine
that they would quibble over the keyword used.

The first patch is a restructure of the documentation for NOT/AND/OR
because IMPLIES is not symmetric about the diagonal and I didn't want it
to stand out like a sore thumb. The second patch is the actual
implementation.

It does not survive a round trip which has precedence with IN being
changed to =ANY, BETWEEN changing to <= and >= (BETWEEN SYMMETRIC is
even worse), etc; so I don't think that is a problem.

Thanks,

--

Vik Fearing

PS: based off of c1c5d28f4a2