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 Th14: tau1.q c= tau1.(p 'or' q)
  proof
    set pq = p 'or' q,np = 'not' p,nq = 'not' q,npq = np '&&' nq;
    tau1.nq c= tau1.npq & tau1.q c= tau1.nq by Th13, Th12;
    then A1: tau1.q c= tau1.npq;
    tau1.npq c= tau1.pq by Th12;
    hence thesis by A1;
  end;
