theorem Satz7p16:
  p,q equiv r,s iff
  reflection(a,p),reflection(a,q) equiv reflection(a,r),reflection(a,s)
  proof
    now
      assume reflection(a,p),reflection(a,q) equiv
        reflection(a,r),reflection(a,s);
      reflection(a,reflection(a,p)) = p & reflection(a,reflection(a,q)) = q &
        reflection(a,reflection(a,r)) = r &
      reflection(a,reflection(a,s)) = s by Satz7p7;
      hence thesis by Lemma7p16a;
    end;
    hence thesis by Lemma7p16a;
  end;
