theorem Th99:
  A\iffB\orC in F & C\iffD in F implies A\iffB\orD in F
  proof
    assume A1: A\iffB\orC in F;
    then A2: B\orC\iffA in F by Th90;
    assume C\iffD in F;
    then C\impD in F & D\impC in F & B\impB in F by Th43,Th34;
    then B\orC\impB\orD in F & B\orD\impB\orC in F by Th59;
    then A\impB\orD in F & B\orD\impA in F by A1,A2,Th92,Th93;
    hence A\iffB\orD in F by Th43;
  end;
