Incremental computation of the value of perfect information in
  stepwise-decomposable influence diagrams
  To determine the value of perfect information in an influence diagram, one
needs first to modify the diagram to reflect the change in information
availability, and then to compute the optimal expected values of both the
original diagram and the modified diagram. The value of perfect information is
the difference between the two optimal expected values. This paper is about how
to speed up the computation of the optimal expected value of the modified
diagram by making use of the intermediate computation results obtained when
computing the optimal expected value of the original diagram.
