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 Satz8p9:
  right_angle a,b,c & Collinear a,b,c implies a = b or c = b
  proof
    assume
A1: right_angle a,b,c & Collinear a,b,c;
    assume
A2: a <> b;
    Collinear b,a,c by A1,GTARSKI3:45;
    hence thesis by A1,A2,Satz8p3,Satz8p8;
  end;
