theorem Satz4p3:
  between a,b,c & between a9,b9,c9 & a,c equiv a9,c9 & b,c equiv b9,c9
  implies a,b equiv a9,b9
  proof
    assume
A1: between a,b,c & between a9,b9,c9 & a,c equiv a9,c9 & b,c equiv b9,c9;
    then c,a equiv a9,c9 by Satz2p4;
    then a,b,c,a IFS a9,b9,c9,a9 by A1,Satz2p8,Satz2p5;
    then a,b equiv b9,a9 by Satz4p2,Satz2p4;
    hence thesis by Satz2p5;
  end;
