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Beer example 4
b(S) - b((V L) - (V S))
The V L are drinker, bar, beer combinations where the beer is liked by the drinker who goes to the bar. The V S are drinker, bar, beer combinations where the beer is served by the bar frequented by the drinker.
The difference and projection gives beers liked by some drinker who visits some bar that does not serve it (i.e. the liked beer).
The whole query is the complement of these beers, thus:
List the beers that are not liked by any drinker who visits a bar that does not serve that beer.
Notes:
Deze query is van Gerjan van der Meer (foute uitwerking van bier query n)