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

theorem Satz2p4:
  a,b equiv c,d implies b,a equiv c,d
  proof
    assume
A1: a,b equiv c,d;
    b,a equiv a,b by GTARSKI1:def 5;
    hence thesis by A1,Satz2p3;
  end;
