:: deftheorem Def1 defines 'nand' BVFUNC25:def 1 :
for p, q being boolean-valued Function
for b₃ being Function holds
( b₃ = p 'nand' q iff ( dom b₃ = (dom p) /\ (dom q) & ( for x being set st x in dom b₃ holds
b₃ . x = (p . x) 'nand' (q . x) ) ) );