 reserve T for Ternary_Boolean_Algebra;
 reserve a,b,c,d,e for Element of T;
 reserve x,y,z for Element of T;

theorem Lemma33: :: Lemma to Theorem 3.3 :: formula 3.31
  Tern (Tern (a,b,c), b, a) = Tern (a,b,c)
  proof
    Tern (Tern (a,b,c), b, a) = Tern (Tern(a,b,c),b,Tern(a,b,b`)) by TRADef
         .= Tern (a,b,Tern(c,b,b`)) by TDis
         .= Tern (a,b,c) by TRADef;
    hence thesis;
  end;
