
theorem
  for x,y being Element of BOOLEAN holds or2a.<*x,y*> = nand2a.<*y,x*>
proof
  let x,y be Element of BOOLEAN;
  thus or2a.<*x,y*> = 'not' x 'or' y by Def4
    .= nand2a.<*y,x*> by Def3;
end;
