theorem Satz6p17:
  p in Line(p,q) & q in Line(p,q) & Line(p,q) = Line(q,p)
  proof
    thus p in Line(p,q)
    proof
      Collinear p,q,p by Satz3p1;
      hence thesis;
    end;
    Collinear p,q,q by Satz3p1;
    hence q in Line(p,q);
    thus Line(p,q)=Line(q,p)
    proof
A2:   Line(p,q) c= Line(q,p)
      proof
        let x be object;
        assume x in Line(p,q);
        then consider y be POINT of S such that
A3:     y = x and
A4:     Collinear p,q,y;
        Collinear q,p,y by A4,Satz3p2;
        hence thesis by A3;
      end;
      Line(q,p) c= Line(p,q)
      proof
        let x be object;
        assume x in Line(q,p);
        then consider y be POINT of S such that
A5:     y = x and
A6:     Collinear q,p,y;
        Collinear p,q,y by A6,Satz3p2;
        hence thesis by A5;
      end;
      hence thesis by A2;
    end;
  end;
