:: deftheorem Def2 defines 'or' BVFUNC_1:def 2 :
for p, q, b₃ being boolean-valued Function holds
( b₃ = p 'or' q iff ( dom b₃ = (dom p) /\ (dom q) & ( for x being object st x in dom b₃ holds
b₃ . x = (p . x) 'or' (q . x) ) ) );