:: deftheorem Def15 defines or3a TWOSCOMP:def 15 :
for b₁ being Function of (3 -tuples_on BOOLEAN),BOOLEAN holds
( b₁ = or3a iff for x, y, z being Element of BOOLEAN holds b₁ . <*x,y,z*> = (('not' x) 'or' y) 'or' z );