reserve A,B,p,q,r,s for Element of LTLB_WFF,
  n for Element of NAT,
  X for Subset of LTLB_WFF,
  g for Function of LTLB_WFF,BOOLEAN,
  x,y for set;

theorem Th21: p 'or' q in tau X implies p in tau X & q in tau X
  proof
    assume p 'or' q in tau X;
    then (('not' p) '&&' ('not' q)) in tau X by Th19;
    then 'not' p in tau X & 'not' q in tau X by Th20;
    hence thesis by Th19;
  end;
