This paper studies a generalization of the a priori implication E less
-than-or-equal-to F, meaning of course EF=E, in the quantum logic P(H)
of all projectors. The new definition can be applied also to new pair
s of events, being based on the indistinguishability of some events wh
en considered from the standpoint of a (fixed) state rho. The mathemat
ical theory of factor Boolean algebras, on which this extension is bas
ed, is clarified.