reserve            S for satisfying_CongruenceSymmetry
                         satisfying_CongruenceEquivalenceRelation
                         TarskiGeometryStruct,
         a,b,c,d,e,f for POINT of S;

theorem Satz2p1: ::EquivReflexive ::GTARSKI1:10
  a,b equiv a,b
  proof
    b,a equiv a,b by GTARSKI1:def 5;
    hence thesis by GTARSKI1:def 6;
  end;
