If the expected committed patch set includes #5 then this becomes a matter for reviewer convenience so never mind. But if its at all possible for #5 to be punted down the road incorporating the eventual "not var" and "not(var)" syntax into #1 as a kind of shim would seem desirable.
why do you need special operator for negation? there is only one use case. It can be solved by \if_not
Not following the thread that closely and the section Robert quoted didn't include "\if_not" as a syntax option. I figured the idea was to limit the number of backslash commands and leave the power in the expression evaluation.
without a expression you can store a negation to variable
I can imagine simple functional only expressions evaluated on client side.
\if not(table_exists('table_name'))
full expressions are not easy implemented without bigger changes in psql parser design - and I don't see any reason why do some too complex there. I would not to replace bash, perl, python or lua.
