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

theorem Satz2p2: ::GTARSKI1:11 ::EquivSymmetric
  a,b equiv c,d implies c,d equiv a,b
  proof
    assume
A1: a,b equiv c,d;
    a,b equiv a,b by Satz2p1;
    hence thesis by A1,GTARSKI1:def 6;
  end;
