:: deftheorem Def15 defines OR4 GATE_1:def 15 :
for a, b, c, d being set holds
( ( ( not a is empty or not b is empty or not c is empty or not d is empty ) implies OR4 (a,b,c,d) = NOT1 {} ) & ( not a is empty or not b is empty or not c is empty or not d is empty or OR4 (a,b,c,d) = {} ) );