• Costs. The state models discussed above can be extended to take into account action costs.
Traditionally, the focus in planning has been on uniform action costs, however, costs may depend
on both actions and states. For example, a fluent t standing for time can be included so that
actions increase t by their time duration. Then,
defining c(a , s) as the duration of a ins, i.e., c(a,s) = (t8 "-t8 )
where sa = next( a, s), a formulation would be obtained in which optimal plans stand for plans
with minimum completion times. In a different way, optimal parallel plans, such as those
computed by
Graphplan and SAT planners, can be defined as well. The issue is how to plan effectively in
such flexible cost structures.