theorem Th12: :: BINARI_3:12

for x, y being Element of BOOLEAN holds
( ( not x 'or' y = TRUE or x = TRUE or y = TRUE ) & ( ( x = TRUE or y = TRUE ) implies x 'or' y = TRUE ) & ( x 'or' y = FALSE implies ( x = FALSE & y = FALSE ) ) & ( x = FALSE & y = FALSE implies x 'or' y = FALSE ) )