6.4 Extensions and Improvements of Graphplan 131
To end this section, let us underline two main features of Graphplan.
1. The mutex relation on incompatible pairs of actions and propositions, and
the weak reachability condition of Proposition 6.4 (see page 123), offer a very
good insight about the interaction between the goals of a problem and about
which goals are possibly achievable at some level.
2. Because of the monotonic properties of the planning graph, the algorithm is
guaranteed to terminate; the fixed-point feature together with the reachability
condition provide an efficient failure termination condition. In particular,
when the goal propositions without mutex are not reachable, no search at all
is performed.
Because of these features and