theorem Satz8p6:
  right_angle a,b,c & right_angle a9,b,c & between a,c,a9 implies b = c
  proof
    assume right_angle a,b,c & right_angle a9,b,c & between a,c,a9;
    then c = reflection(b,c) by GTARSKI3:55;
    hence thesis by GTARSKI3:104;
  end;
