
theorem
  for x,y being Element of BOOLEAN holds  and2a.<*x,y*> = nor2a.<*y,x*>
proof
  let x,y be Element of BOOLEAN;
  thus and2a.<*x,y*> = 'not' ('not' 'not' x 'or' 'not' y) by Def1
    .= nor2a.<*y,x*> by Def6;
end;
