>a Boolean algebra (or in operational terms, ~ *is* complement for those >propositions of the form ~a, making ~a+~~a=1, and ~a~~a=0, and ~~~a = >~~a). Oops, I was asleep at the wheel here. ~a+~~a and ~a~~a are only 1 and 0 respectively in the ~~ retract of H to a Boolean, not in H itself, sorry about that. ~~~ = ~ is fine. Vaughan