theorem Th84:
  B\or\notC\andC\impB in F
  proof
    (C\and\notC\impB) in F & \notC\andC\impC\and\notC in F by Th50,Def38; then
    \notC\andC\impB in F & B\impB in F by Th34,Th45; then
    B\or\notC\andC\impB\orB in F & B\orB\impB in F by Th59,Th52;
    hence B\or\notC\andC\impB in F by Th45;
  end;
