gf-3.1.6: examples/SUMO/axioms/GeographyHiO.gft
SymmetricRelation ? (\x,y -> oppositeDirection x y) IrreflexiveRelation ? (\x,y -> oppositeDirection x y)
SymmetricRelation ? (\x,y -> oppositeDirection x y) IrreflexiveRelation ? (\x,y -> oppositeDirection x y)