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

theorem Satz2p5:
  a,b equiv c,d implies a,b equiv d,c
  proof
    assume
A1: a,b equiv c,d;
    c,d equiv d,c by GTARSKI1:def 5;
    hence thesis by A1,Satz2p3;
  end;
