t = 0; s =  :
WHLE
rrrr BEGIN
e ecute simultaneously all operators such that i s =
and the preconditions of operator   are true; c =  such that si =
..

st =  such that s i l
..

s II
IF fact   is true THENs = sT ELSEs = sF;
t = t +  ;
END
igure    Procedure for e ecuting a plan represented plan. The word simultaneously" refers to the re uirement that the set of operators   with true preconditions and operators are e ecuted. Note that by the uni ueness schemata . . . the  in  s,i,  s,i and   s  i    is uni  uely determined by   .

## 4.2.2 Plans As Sequences Of Sets Of Iterated Operators

The plans in the previous section are very general, and the number of even relatively small plans can be very high, which makes finding plans and determining the ine istence of plans di  cult.    any conditional planning problems have solutions as more restricted and computationally less e  pensive forms of plans.

The plans discussed in this section have an internal state like the plans of the previous section, and each state is associated with a set of operators enabled in the state, but the state transitions are more restricted. The plan stays in the same state as long as some of the operators enabled in it are applicable; that is, as long as there is an enabled operator the preconditions of which are true and some of the postconditions are false.

hen there are no such operators, an unconditional transition to a uni_ue successor state is made.

Not all problem instances that have a solution as a plan of the form discussed in   ection
. have a solution as a plan of the form discussed in this section.

Example 4.3 onsider the following operators.

0 Bob-has-1000DM
Bob-has-1000DM   Bob-in-Kyoto Bob-has-1000M   Bob-in- Paris Bob-has-1000DM
food-in-Kyoto   Bob-in-Kyoto   Bob-has-5DM
food-in-Paris  Bob-in-Paris  Bob-has-5DM
nitially e actly one of the facts food-in-Kyoto and food-in-Paris is true, all of the facts Bob-has-1000M Bob-has-5DM and Bob-hungry are true, and the rest of the facts are false. The goal is   Bob-hungry.

ith plans of the form discussed in  ection  .  .  the goal can be achieved as follows.

nitially a transition to one of the internal states and is made, depending on which of