theorem Lemma5p12p2:
  a,b <= c,d implies a,b <= d,c
  proof
    assume
    a,b <= c,d;
    then ex x be POINT of S st between a,b,x & a,x equiv c,d by Satz5p5;
    hence thesis by Satz2p5,Satz5p5;
  end;
