theorem Satz7p10:
  reflection(a,p) = p iff p = a
  proof
    hereby
      assume reflection(a,p) = p;
      then Middle p,a,p by DEFR;
      hence p = a by GTARSKI1:def 10;
    end;
    assume p = a;
    then Middle p,a,p by Satz3p1,Satz2p1;
    hence thesis by DEFR;
  end;
