where pa is the probability that a is true, and pb is the probability that b is true.
We use the name beland(q) to indicate that this is the belief value (probability)
that results from an AND combination.
This means that the AND combination of evidence is computed by simply multiplying
the probabilities of the parent nodes. If one of the parent probabilities
is low (or zero if smoothing is not used), then the combination will have a low
probability. This seems reasonable for this type of combination. We can define a
number of other combination operators in the same way. If a q node has n parents
with probability of being true pi, then the following list defines the common
operators: