theorem Th103:
  A\impB in F & C\impD in F implies B\impC\imp(A\impD) in F
  proof assume
    A\impB in F & C\impD in F;
    then B\impC\imp(A\impC) in F & A\impC\imp(A\impD) in F by Th101,Th102;
    hence thesis by Th45;
  end;
