Deductive Planning with Inductive Loops
Martin Magnusson and Patrick Doherty
Department of Computer and Information Science
Linköping University, 581 83 Linköping, Sweden
marma@ida.liu.se,patdo@ida.liu.se

Abstract
Agents plan to achieve and maintain goals. Maintenancethat requires continuous action excludes the representationof plans as finite sequences of actions. If there is no upperbound on the number of actions, a simple list of actions would
be infinitely long. Instead, a compact representation requires
some form of looping construct. We look at a specific temporally extended maintenance goal, multiple target video surveillance, and formalize it in Temporal Action Logic. The
logic’s representation of time as the natural numbers suggests
using mathematical induction to deductively plan to satisfy
temporally extended goals. Such planning makes use of asound and useful, but incomplete, induction rule that compactly represents the solution as a recursive fixpoint formula.
Two heuristic rules overcome the problem of identifying asufficiently strong induction hypothesis and enable an automated solution to the surveillance problem that satisfies thegoal indefinitely.

Introduction
Research in cognitive robotics has produced autonomous
robots such as unmanned aerial vehicles (UAVs) that can
carry out complex tasks without human intervention (Doherty 2004; Doherty et al. 2004; Doherty &Rudol 2007).
But many natural applications are associated with automatedplanning problems that are beyond the representational capabilities of most automated planners. While one could circumvent this problem by providing ready-made plans forthese problems, doing so sacrifices flexibility since suchplans will not work in unforeseen circumstances.
Consider e.g. a surveillance task. The goal is to continually get information or video footage of a number of target
locations. Such a goal is not achieved, but rather maintained
indefinitely or until the surveillance task is aborted. Viewing plans as sequences of actions would result in an infinite
number of actions, flying back and forth to keep informationon the targets current. Any workable solution must instead
view plans as simple programs that make use of some formof loop construct. As a consequence, planning must correspond to a simple form of program synthesis.
Program synthesis is most often considered in a deductive framework as the result of a constructive proof of the
Copyright c 2008, Association for the Advancement of Artificial
Intelligence (www.aaai.org). All rights reserved.

program’s specification, or the planning goal. Previous research (Magnusson 2007) has investigated deductive planning in Temporal Action Logic (TAL) (Doherty &Kvarnström 2007) where time is explicitly represented by the natural numbers. Achievement goals are then expressed as theexistence of a time point at which the goals are satisfied.
Maintenance goals, in contrast, must hold at all time points:∀t [Goals(t)]This formulation strongly suggests the use of mathematicalinduction over the natural number time line. The base caseproof would result in initialization actions that form a set-up
phase in the task’s solution plan. This would be followed byan induction step proof to produce actions that must be performed iteratively. Finally, applying the principle of mathematical induction would append an infinite chain of copiesof the iterated actions to the initialization actions, thereby
closing the loop.
However, some difficulties must be overcome before thisintuition can be realized. First, mathematical induction usually proceeds in one step increments. One proves that if the
proposition holds for x, it will hold for x + 1. This is inconvenient when multiple actions with non-unit durations are
involved in the induction step proof. We solve this problem
by using an induction rule parameterized on the induction
step size s. The choice of a value for the step size s is delayed until the proof of the induction step is completed.
Second, even if a goal formula is provable using induction, it might not be inductive by itself (Manna &Waldinger
1987). In other words, assuming that the formula holds at x
might not provide sufficient grounds for a proof that it holds
at x+s. Instead, one needs to find a “stronger” induction hypothesis that is inductive. The proof of the stronger formulathen entails the original formula. Finding an inductive variant of the goal formula is a serious difficulty, and the usual
solution is to delegate this task to a human. We add heuristic goal strengthening rules that guide the proof process into
trying promising strengthened variants of the original formula.
Third, the resulting plan contains a loop and loops can
not in general be expressed in first-order logic. We add agreatest fixpoint operator to our logic that admits a compact
representation of loops in terms of recursive expressions thatcan be executed by the agent.

After briefly surveying related work and introducing theabove paraphernalia, we present a solution to the surveillance task produced by an automated theorem prover basedon natural deduction.

Related Work
While most automated planning systems do not have the capability to generate plans with loops, nor do they have plan
representation languages expressive enough to encode solution plans with loops, we discuss some notable exceptions
below.
One such exception is Manna and Waldinger’s (1987)
plan theory. It is based on the Situation Calculus and uses induction over well-founded relations, such as the blocksworld
relation above(x, y). They apply an automated tableau theorem prover to the task of clearing a block, i.e. ensuring thatno block is above the cleared block. A block can be cleared
e.g. by clearing the block that is directly above, and thenmoving that block to the table. In constructing this plan
Manna and Waldinger end up with two conditions that are
unprovable without a strengthening of their induction hypothesis. However, suggestions for how such strengthening
could be automated are not provided.
Cresswell, Smaill, and Richardson (2000) extend previous deductive planning approaches based on intuitionistic
linear logic. A recursive data type is introduced along withan induction rule for it. This is linked to the deductive synthesis of recursive plans using concepts from type theory.
Automation is provided in Lolli (Hodas &Miller 1994), alogic programming language based on intuitionistic linear
logic, where heuristics help at “difficult choice points” suchas when choosing what induction rule to use. One stepin their proof requires an induction hypothesis strengthening, generalizing their goal of inverting a blocksworld tower,
which they state is “amenable to automation”. Whether they
do automate it is not made clear by the paper.
Stephan and Biundo (1993) present a deductive planning
framework based on Dynamic Logic. Although they treat“recursively defined notions”, again citing the blocksworld
above relation, to the best of our knowledge they do not generate solution plans with recursion or loops.
In later work (Stephan &Biundo 1996) they introduce aninterval-based modal temporal logic called Temporal Planning Logic, which can be used to formulate plans that contain recursive procedures. However, these are “prefabricatedgeneral solutions [that] are adapted to special problems”.
There is no attempt to generate such plans since they claimthat this “cannot be carried out in a fully automatic way”. Ifthey are taken to mean that there can be no complete algorithm for generating recursive procedures it is clear that they
are correct since plans with recursion are Turing complete.
However this should not stop us from attempting to develop
fully automated synthesis for interesting special cases thatare useful for large classes of real world problems.
Similarly, Koehler’s PHI planner (Koehler 1996) canwork with plans that contain complex control structures likeconditionals and loops. But loop constructs are reused or
modified from existing plans, not generated from scratch.

Bundy et. al. (2006) argue that “induction is needed whenever some form of repetition is required”. They present algorithms for constructing new induction rules on the fly insteadof relying on a fixed set of predefined rules. Their results areapplied to the problem of automatic program synthesis froma specification rather than to the related problem of planning
robot actions.
A non-inductive strategy is pursued by Levesque (2005)
in his KPLANNER. It explores an interesting middle groundbetween our direct synthesis of loops and “traditional noniterative planning”. The planner works by placing an upperbound on the planning parameter, e.g. the time line in our
surveillance example. The bounded problem can be solved
using classical planning techniques. Next, the planner identifies action sequences that could correspond to unwinded
loops and winds them up so that the result is a compact planwith loops. Levesque shows how this method is guaranteed
correct for certain classes of planning problems, though it
seems to us that an inductive formulation of planning withloops is more straightforward.
More different still are systems based on model checking, such as the Cooperative Intelligent Real-Time Control
Architecture (CIRCA) (Musliner, Durfee, &Shin 1993) andthe MBP planner (Cimatti et al. 2003). While they allow
cyclic solutions, they also rely on an explicit state space
representation, which we want to avoid. Even if manageable state spaces can be constructed for many benchmark
planning problems, we would like to consider planning ina more general setting. An autonomous agent trying to satisfy a goal will have a large body of background knowledge
about the world. An explicit representation of the resulting
state space would result in a state explosion. One may attempt to circumvent this problem by planning in a limited
state representation involving only those fluents that are relevant to the satisfaction of the given goal. But this begs thedifficult question of how one would know what fluents are
relevant before the search for a solution has even begun.

Inductive Proof
Manna and Waldinger (1987) state that “In proving a given
theorem by induction, it is often necessary to prove astronger, more general theorem, so as to have the benefitof a stronger induction hypothesis”. Pnueli (2006) provides
a simple example:∀x [∃y [1 + 3 + 5 + · · · + (2x − 1) = y 2 ]](1)

Proving Formula 1 using induction over the natural numbers
is not possible by using the body of the universal quantifier
as the induction hypothesis. Instead one must find a stronger
inductive hypothesis, the proof of which entails the original
formula.
Now, consider the surveillance task mentioned above.
Suppose that we need to have up-to-date image data, at most
15 minutes old, from three locations. Our UAV continually
records video so visiting a location suffices to obtain image
data from it. We state that the location feature loc for a UAV
uav1 assumes, e.g., the value pos(160, 500) at some time
point y by value(y, loc(uav1 )) = pos(160, 500). If time
points represent seconds then we can express the fact thatthe visit takes place no more than 15 minutes from the current time point x by 0 ≤ y − x < 900. In the surveillancetask the goal is to always have recent information, i.e. theabove condition is satisfied at all time points:∀x [∃y [value(y, loc(uav1 )) = pos(160, 500) ∧0 ≤ y − x < 900]](2)

Formula 2, unfortunately, is too weak as an induction hypothesis for the same reason as Formula 1. Assuming only
the existence of a visiting time y within 900 seconds from x
does not leave any time for flying around while still making
sure the condition holds at x + s. In the worst case, 899 seconds have already passed since y and the UAV would needto visit pos(160, 500) again the very next second.
The task of finding a strengthening of the goal formula
is often left to a human. E.g., Pnueli’s Formula 1 is easily proved if the existential quantifier is removed and thevariable y replaced by the variable x. This stronger formula entails the original by existential generalization. But
an autonomous UAV cannot depend on human ingenuity tohelp solve its planning problems. Moreover, as previously
mentioned, there can be no complete proof method for induction in general. Our solution is instead the addition ofgoal strengthening rules. These are proof rules that suggest
stronger versions of goals of certain commonly occurringforms. Their theoretical redundancy but practical significance, in guiding the proof search, make these rules heuristic
in nature.

Regularity Heuristic
If the robot knew the time point at which a location waslast visited it would have more freedom in planning its next
visit. E.g., if the UAV decides on a regular flight schedule
it would, at any given time point, know how much time had
passed since its last visit to the target location. We can capture this intuition as a heuristic goal strengthening rule. The
following rule guides a backward chaining proof search, andshould therefore be read backwards from the conclusion to
the premise. If we are trying to prove that there always exists
a time point y within n time points where P holds, then wemight instead try to prove that there is some regular schedule of length s no larger than n such that P always holds
for each repetition i of the schedule, with some offset m lessthan n:(Regularity Heuristic)∃s [s ≤ n ∧ ∃m∀i [P (is + m) ∧ 0 ≤ m < n]]∀x∃y [P (y) ∧ 0 ≤ y − x < n]Theorem 1. The regularity heuristic is sound.
Proof. Assume the premise and that x is an arbitrary number. Choose the smallest i such that is + m ≥ x. By assumption P (is + m) holds, and since s ≤ n and m < n the
distance from x to is + m must be less than n. Thus for any
x there exists a y within n time points (namely is + m) suchthat P (y) holds.

Applying this rule to Formula 2 produces a new goal:∃s [s ≤ 900 ∧∃m [∀i [value(is + m, loc(uav1 )) = pos(160, 500) ∧0 ≤ m < 900]]]

Synchronization Heuristic
Of course, as long as there is only one location to monitor, the UAV could get by with the trivial solution of justhovering over it. More interesting behaviour is necessary
if up-to-date information from several different locations isrequired:∃s [s ≤ 900 ∧∃m [∀i [value(is + m, loc(uav1 )) = pos(160, 500) ∧0 ≤ m < 900]]] ∧∃s [s ≤ 900 ∧∃m [∀i [value(is + m, loc(uav1 )) = pos(160, 680) ∧0 ≤ m < 900]]] ∧∃s [s ≤ 900 ∧∃m [∀i [value(is + m, loc(uav1 )) = pos(400, 680) ∧0 ≤ m < 900]]](3)
While each sub-goal could be planned for separately, thiswould most likely result in a very inefficient proof search.
The UAV would first construct plans that work for each location in isolation and backtrack if the combined plan requires it to visit two locations simultaneously, again reconsidering each plan in isolation. A better strategy would be to
assume that all of the visits are part of the same schedule andconsider them simultaneously. This strategy corresponds toanother goal strengthening heuristic. If trying to prove theexistence of several different schedules with independent iteration variables we may instead synchronize them into one
schedule and iteration:(Synchronization Heuristic)∃s∀i [P1 (s, i) ∧ · · · ∧ Pn (s, i)]∃s1 ∀i1 [P1 (s1 , i1 )] ∧ · · · ∧ ∃sn ∀in [Pn (sn , in )]Theorem 2. The synchronization heuristic is sound.
Proof. Assume the premise. Distribute the universal quantifier over the conjunction. Since the entire conjunction holds
for some s, each conjunct must hold for some s, as in the
conclusion.
By moving the m variables of Formula 3 into the prefix,
renaming them apart, and applying the above heuristic, wearrive at a new proof goal:∃m1 , m2 , m3 , s [∀i [s ≤ 900 ∧value(is + m1 , loc(uav1 )) = pos(160, 500) ∧0 ≤ m1 < 900 ∧value(is + m2 , loc(uav1 )) = pos(160, 680) ∧0 ≤ m2 < 900 ∧value(is + m3 , loc(uav1 )) = pos(400, 680) ∧0 ≤ m3 < 900]](4)

Induction Rule
The goal strengthening heuristics enable the formulation of
an induction rule for planning with loops by introducing initialization actions I in the base case and loop step actions A
in the induction step, as further explained below:(Induction Rule)
I → P (0)∀x [P (x) ∧ A(x) → P (x + s)]
I ∧ νX(x)[A(x) ∧ X(x + s)](0) → ∀i [P (is)]The initialization actions I make sure that the induction basecase holds. The actions A inside the loop depend on the
induction variable x and make sure the goal P holds for the
next loop iteration at x + s.
When the induction principle is applied the loop step actions need to be repeated indefinitely. This could be expressed in a declarative manner using a universal quantifier.
But since the purpose of the formula is to function as an executable plan for the UAV, we express it in a constructive
manner by introducing a greatest fixpoint operator ν. The
expression νX(x)[A(x) ∧ X(x + s)] defines a new relation
X with a time point argument x at which the loop step actions A hold. Moreover, X itself holds at x + s, thereby
setting up a recursive iteration. Finally, the loop expression
is initialized by providing the base case time point 0 as its
argument. Note that we assume s > 0 and i ≥ 0 since s
corresponds to the loop duration and i to loop repetitions,none of which make sense for negative values.
The fixpoint loop together with the initialization actions
ensure only that P holds at any multiple i of s. However,
the goal strengthening heuristics have already put the surveillance goal into this form in Formula 4. The original goal,
covering all time points as in Formula 2, is a guaranteed consequence due to the heuristics’ soundness. Soundness of theinduction rule will be proved using the following lemma.
Lemma 3. The induction rule fixpoint formula corresponds
to occurrences of actions A at regular intervals starting at 0
and spaced by s, i.e.:
νX(x)[A(x) ∧ X(x + s)](0) ⇔A(0) ∧ A(s) ∧ A(2s) ∧ · · ·Proof. The Knaster-Tarski fixpoint theorem states the existence of a greatest fixpoint of a formula F , defined by
Kleene’s iterative construction as the limit of the generalized conjunction bounded by ω:
νX(x)[F (X(x))] ⇔^F i (true)i<ω

where F 0 is the identity function and F (i+1) is F with occurrences of X(x) replaced by F i instantiated with x. In
our case we have F = A(x) ∧ X(x + s) and:F 0 = true
F 1 = F (F 0 ) = A(x) ∧ true
F 2 = F (F 1 ) = A(x) ∧ A(x + s)

F 3 = F (F 2 ) = A(x) ∧ A(x + s) ∧ A(x + s + s)..
.
We see that the form of F i is:
F i = A(x + 0s) ∧ A(x + 1s) ∧ · · · ∧ A(x + is)Clearly F i subsumes F (i−1) for every i in the generalized
conjunction above. What remains is then F ω , which we
write in the form of an infinite conjunction:
F ω = A(x) ∧ A(x + s) ∧ A(x + 2s) ∧ · · ·By instantiating the free variable x with 0 we arrive at theright hand side of Lemma 3:
A(0) ∧ A(s) ∧ A(2s) ∧ · · ·Theorem 4. The induction rule is sound.
Proof. The proof that the induction rule conclusion follows
from the premises proceeds by mathematical induction overthe size k of the domain {0, . . . , k} of the natural number i.
In the base case i = {0}. Given that the premises hold,assume the antecedent of the conclusion implication, including I. This, together with the first premise, immediately
produces P (0) which can be rewritten P (0s). Since 0 is the
only value i may assume we have P (is) for all i.
As induction hypothesis we assume that the induction rule
is sound for i = {0, . . . , k} and show that it holds for
i = {0, . . . , k, k + 1}. Thus the conclusion follows fromthe premises for all i up to and including k and what remains is to show that it follows for i = k + 1, i.e. to showthat I ∧ νX(x)[A(x) ∧ X(x + s)](0) → P ((k + 1)s). Assume the antecedent. The induction hypothesis holds fori = k and gives us P (ks). According to Lemma 3 we haveA(0) ∧ A(s) ∧ A(2s) ∧ · · · ∧ A(ks) ∧ · · · . Taken together
we have P (ks) ∧ A(ks) which, from the second induction
rule premise, produces P (ks + s), i.e. P ((k + 1)s). We
have now proved the consequent of the conclusion implication for all possible values of i and the universally quantified
conclusion follows.
By the principle of mathematical induction the rule holds
for all possible sizes of the domain of i.

Bundy’s Criteria for Heuristic Rules
While the heuristics and induction rule can not be complete,
one would wish for them to cover some useful and commonly occurring cases. At the same time, one wants to avoid
creating a set of specialized ad hoc rules that solve only aparticular problem. Bundy (1988) considers desirable properties of proof plans, which are used to guide the search fora proof. Our heuristic rules are clearly related to the conceptof proof plans, and evaluating them with regard to Bundy’s
properties can help explain their usefulness. Specifically,
Bundy’s generality and expectancy (why one expects therule to work) properties are relevant and we examine eachof the rules with respect to these properties below.

Regularity Heuristic The regularity heuristic is applicable to any condition P for which only a limited amount oftime n should pass before it is satisfied again. The heuristic suggests that these can be accomplished by deciding on
a fixed period s and offset m at which to satisfy them. Reallife examples include e.g. a bus servicing a stop every 20
minutes, or backing up data each week on Friday.
Synchronization Heuristic The synchronization heuristic
is applicable when several iteration periods si are involved
and suggests synchronizing them with each other by using acommon period s. This is especially useful when the same
resource is involved in the different tasks. E.g., if the UAV’s
surveillance of each location is planned in isolation there
is great risk that the different schedules will conflict whenmerged. By synchronizing the iteration periods and organizing all the tasks in a single schedule one readily detects andresolves attempts of conflicting resource use.
Induction Rule The heuristics taken together with the induction rule provide the high-level structure for solving aclass of commonly occurring temporally extended planninggoals by planning actions in an iterative loop. Specifically,
any tasks where some general constraint is satisfied by periodic actions are candidates. Such tasks are plentiful, with
examples from different domains such as autonomous surveillance or a housekeeper robot watering the plants every
day to keep them alive. In fact, much of your day probably
consists of loops that are designed to keep some parameterwithin an acceptable range through regular action, e.g. mental alertness through regular sleep and brain caffeination.

Automating Planning with Loops
The question of automation still remains. Previous work on
deductive planning in Temporal Action Logic used a compilation of TAL formulas into Prolog programs (Magnusson
2007). But Prolog’s limited expressivity makes it inadequate
for our present purposes. Instead, our current work utilizes
Pollock’s natural deduction with quantifier-free form (Pollock 1999) that replaces the somewhat cumbersome quantifier introduction and elimination rules found in most natural
deduction systems with unification.
Our theorem prover implementation is named ANDI, foraugmented natural deduction intelligence. Its rules are divided into forward and backward rules. Forward rules are
triggered whenever possible and are designed to converge
on a stable set of conclusions to avoid generating new inferences forever. Backward rules, in contrast, are used tosearch backwards from the current proof goal and thus exhibit goal direction. Combined, the result is a bi-directional
search for proofs.
We make non-monotonic reasoning and planning possible through a “natural abduction” proof rule. Relations froma set of abducibles are allowed to be assumed rather thanproven, as long as doing so does not lead to inconsistency. It
is well known that the problem of determining consistencyof a first-order theory is not even semi-decidable. Our theorem prover relies on its forward rules to implement an incomplete consistency check. If an abductive assumption andthe triggering forward rules produce a contradiction, thenthe prover needs to backtrack and cancel some assumptionthat participates in the proof of the contradiction. If an inconsistent assumption remains undetected the resulting implication for a goal formula P would have the form ⊥ → P
and thus, while consistent, be tautological and void.

The Surveillance Problem
Given the heuristics and the induction rule, the only thingthat ANDI is missing to solve the surveillance problem is aformalization of an action for flying between locations. A
version that takes the distance between the locations into account is presented below. Note that the universal quantifiers
for agents, time, and locations are dropped in quantifier-freeform, and the corresponding variables a, t, l1 , and l2 are
prefixed by question marks:(-> (and (= (value ?t (loc ?a)) ?l1)
(occurs ?a ?t (fly ?l1 ?l2)))(= (value (+ ?t (dist ?l1 ?l2))(loc ?a))?l2))

The surveillance goals of the form (2) are put into quantifierfree form through skolemization. Existentially quantified
variables are replaced by skolem constants and functions,prefixed by a $. E.g., since the existentially quantified yvariables depend on the universally quantified x variables
they result in skolem functions of the form ($y ?x). The
goal is then:(and (= (value ($y1 ?x1) (loc uav1))(pos 160 500))(<= 0 (- ($y1 ?x1) ?x1))(< (- ($y1 ?x1) ?x1) 900)(= (value ($y2 ?x2) (loc uav1))(pos 160 680))(<= 0 (- ($y2 ?x2) ?x2))(< (- ($y2 ?x2) ?x2) 900)(= (value ($y3 ?x3) (loc uav1))(pos 400 680))(<= 0 (- ($y3 ?x3) ?x3))(< (- ($y3 ?x3) ?x3) 900))

Finally, we need to know the initial location of the UAV:(= (value 0 (loc uav1)) (pos 160 500))

Provided this input, the ANDI theorem prover automaticallygenerates the following plan:(and (occurs uav1 0 (fly (pos 160 500)
(pos 160 680)))(occurs uav1 180 (fly (pos 160 680)
(pos 400 680)))((gfp (rel ?x)(and (occurs uav1 (+ ?x 420)
(fly (pos 400 680)
(pos 160 500)))(occurs uav1 (+ ?x 720)
(fly (pos 160 500)
(pos 160 680)))(occurs uav1 (+ ?x 900)
(fly (pos 160 680)
(pos 400 680)))(rel (+ ?x 720)))) 0))

Number of Targets
10
20
30

CPU Time in Seconds
1
5
17

Table 1: Time spent by ANDI (on average over three runs)
solving different size surveillance problems on a Pentium M
1.8 GHz laptop with 512 MB of RAM.
ANDI integrates constraint solvers and rewrite rules into theproof process. In this case an inequality constraint solver determined the value 720 for s and rewrite rules were used e.g.
to simplify instantiated arithmetic expressions, most importantly the distance function between location coordinates.
Very large (or infinite) domains such as numeric time andcoordinate spaces are no problem for the backward chaining
search, which picks relevant values from the huge set of possibilities by unification with the goal or a sub-goal derived
from the goal. Moreover, the most general unifier does notnecessarily instantiate variables and can thereby often avoid
early commitment to choices that might lead to unnecessary
backtracking. All of these factors contribute to the theorem
prover’s practical usefulness in solving real world size planning problems, as exemplified by Table 1.

Conclusion
Many interesting problems require planning with loops, forwhich there can be no complete algorithm. We considered a useful subset of temporally extended goals that canbe solved by regular repeated action. These problems canbe solved using a proof rule for mathematical induction together with a couple of heuristic rules that put the goal
into a form where the induction rule is applicable. Natural deduction conveniently captures this relatively complex
proof strategy through its extensible set of proof rules since,
when a planning goal matches a rule’s conclusion, the rule’s
premises functions as a proof agenda to follow. For the induction rule, the result is a fixpoint loop solution that compactly represents an infinite plan.
We applied the methodology to a UAV surveillance problem that was quickly solved by an automated natural deduction theorem prover. By concentrating on surveillance we
could ignore, for now, the frame problem since no reasoning
about persistence was involved. Furthermore, the temporally extended goal was solved by a simple application ofmathematical induction over the TAL time line since it consists of the natural numbers. However, there are clearly more
complex classes of planning problems, requiring solutionsof an iterative nature, where the same methodology could beattempted. Consider e.g. a logistics mission where the goal
is to distribute supply crates. The crates’ current locations
and target destinations could be represented as well-founded
sets, thereby making induction applicable. A plan that loops
over crates would represent a constant size solution, regardless of the number of crates that need to be distributed. We
believe that this ability to utilize complex proof rules, suchas induction, to find compact plans in highly expressive for-malisms, such as loops in fixpoint logic, provides logicalplanning with a potential to scale up to realistically sized
problems.

Acknowledgements
We thank Andrzej Szalas for fruitful discussions about fixpoints.
This work is supported in part by the Swedish ResearchCouncil VR grant 2005-3642, the National Aeronautics Research Program NFFP04 S4203, CENIIT, and the Strategic
Research Center MOVIII, funded by the Swedish Foundation for Strategic Research, SSF.

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