:: deftheorem defines are_orthogonal GTARSKI4:def 6 :
for S being non empty satisfying_Tarski-model TarskiGeometryStruct
for a, b, c, d being POINT of S holds
( are_orthogonal a,b,c,d iff ( a <> b & c <> d & are_orthogonal Line (a,b), Line (c,d) ) );