theorem Th12: tau1.p c= tau1.('not' p)
  proof
    set np = 'not' p,f=TFALSUM;
A1: tau1.p c= {np} \/ tau1.p by XBOOLE_1:7;
    {np} \/ tau1.p c= {np} \/ tau1.p \/ tau1.f &
    tau1.np = {np} \/ tau1.p \/ tau1.f by XBOOLE_1:7, Def4;
    hence thesis by A1;
  end;
