reserve S for non empty satisfying_Tarski-model
              TarskiGeometryStruct,
        a,b,c,d,c9,x,y,z,p,q,q9 for POINT of S;
reserve              S for satisfying_Tarski-model TarskiGeometryStruct,
        a,a9,b,b9,c,c9 for POINT of S;

theorem Satz8p3:
  right_angle a,b,c & a <> b & Collinear b,a,a9 implies right_angle a9,b,c
  proof
    assume that
A1: right_angle a,b,c and
A2: a <> b and
A3: Collinear b,a,a9;
    now
      b,c equiv reflection(b,b),reflection(b,c) by GTARSKI3:105;
      hence b,c equiv b,reflection(b,c) by Lem01;
    end;
    hence thesis by A1,A2,A3,GTARSKI3:53;
  end;
