:: deftheorem Def20 defines xor3 TWOSCOMP:def 20 :
for b₁ being Function of (3 -tuples_on BOOLEAN),BOOLEAN holds
( b₁ = xor3 iff for x, y, z being Element of BOOLEAN holds b₁ . <*x,y,z*> = (x 'xor' y) 'xor' z );