Sometimes, unfortunately, it may happen that we
find out that not all necessary sets have been evaluated.
There has been a failure in the sampling if all frequent
sets are not found in one pass, i.e., if there is a frequent
set X in F(r, mixfr) that is not in SUfW (S). A mzss
is a frequent set Y in F(r, min-fr) that is in BP(S).