theorem Lm4p13p1:
  for S being satisfying_CongruenceSymmetry
              satisfying_CongruenceEquivalenceRelation
              TarskiGeometryStruct
   for a,b,c,a9,b9,c9 being POINT of S st a,b,c cong a9,b9,c9 holds
   b,c,a cong b9,c9,a9
  proof
    let S be satisfying_CongruenceSymmetry
             satisfying_CongruenceEquivalenceRelation
             TarskiGeometryStruct;
    let a,b,c,a9,b9,c9 be POINT of S;
    assume
A1: a,b,c cong a9,b9,c9;
    then b,a equiv a9,b9 & c,a equiv a9,c9 by Satz2p4;
    hence b,c,a cong b9,c9,a9 by A1,Satz2p5;
  end;
