:: deftheorem Def13 defines NOR3 GATE_1:def 13 :
for a, b, c being set holds
( ( not a is empty or not b is empty or not c is empty or NOR3 (a,b,c) = NOT1 {} ) & ( ( not a is empty or not b is empty or not c is empty ) implies NOR3 (a,b,c) = {} ) );