A THREE-LEVEL MODEL FOR PLAN EXPLORATION 
Lance A. Ramshaw 
Department of Computer Science 
Bowdoin College 
Brunswick, ME 04011 
Internet: ramshaw@polar.bowdoin.edu 
ABSTRACT 
In modeling the structure of task-related discourse 
using plans, it is important to distinguish between 
plans that the agent has adopted and is pursuing 
and those that are only being considered and ex- 
plored, since the kinds of utterances arising from 
a particular domain plan and the patterns of ref- 
erence to domain plans and movement within the 
plan tree are quite different in the two cases. This 
paper presents a three-level discourse model that 
uses separate domain and exploration layers, in 
addition to a layer of discourse metaplans, allow- 
ing these distinct behavior patterns and the plan 
adoption and reconsideration moves they imply to 
be recognized and modeled. 
DISCOURSE MODEL LEVELS 
In task-related discourse, much of the discourse 
structure derives directly from the task structure, 
so that a model of the agent's plans can serve as 
a useful discourse model, with discourse segment 
boundaries mapping to sub-plan boundaries and 
the like. This simple model works well in appli- 
cations like expert-apprentice dialogues, where a 
novice agent is currently pursuing a single domain 
plan. Since the discourse tracks the domain plan 
so closely in such cases, it is fairly easy to make the 
links between the agent's queries and the relevant 
domain plans. 
But this single-level model is not rich enough 
to handle phenomena like clarification subdia- 
logues or plan revision, as seen in the work of 
Litman, Carberry, and others. Litman's model 
\[Lit85, LA87, LA90\] employs a stack of discourse 
mctaplans on top of the base domain plan, produc- 
ing a two-level model that can handle clarification 
subdialogues and other discourse phenomena that 
go beyond the domain plans. Carberry \[Carg0\] 
adds an independent stack of discourse goals, for 
similar reasons. 
In earlier work \[Ram89a\], I explored the use 
of a different kind of metaplan to model what I 
called the problem-solving level, where the agent 
is exploring possible plans, rather than pursuing 
an adopted one. Such plan exploration contexts, 
which can include comparison between alternative 
plans or consideration of plans in hypothetical cir- 
cumstances, are quite different from adopted do- 
main plan execution contexts, both in terms of the 
reference patterns to domain plans and in terms 
of the kinds of utterances that are generated. 
This paper describes an effort to combine these 
earlier approaches into a three-level model where 
the discourse metaplans can be rooted in either the 
exploration or domain plan levels, so that both 
kinds of plan-related behavior can be modeled. 
Such a model can capture the differences between 
the plan exploration level and the domain level 
in terms of the appropriate plan recognition and 
query generation processes, thus broadening the 
range of discourse phenomena that can be mod- 
eled. It also allows us to model shifts between lev- 
els, as the agent explores, adopts, and reconsiders 
particular plans. 
THE NATURE 
OF PLAN EXPLORATION 
Cohen and Levesque \[CLg0\] have recently pointed 
out the theoretical problems that arise, while us- 
ing a planning system to model a rational agent, 
from failing to distinguish between a system's 
plans and its intentions, since agents frequently 
form plans that they never adopt. It is this same 
distinction that motivates the division proposed 
here of the domain-plan-related portion of the dis- 
course model into separate levels representing first 
those domain plans and goals which the agent has 
adopted and is pursuing, here called the domain 
39 
layer, and second those which the agent is explor- 
ing but has not adopted, the exploration layer. 
While the same domain plans give structure to 
these two levels, the resulting discourse phenom- 
ena including the space of relevant queries based 
on those plans and the patterns of references to 
plans in the plan tree are quite different. 
QUERY TYPES 
One clear difference can be seen in the content of 
utterances arising on those different layers from 
the same underlying domain plan. For example, 
in a banking context, consider these two queries: 
What is the interest rate on your pass- 
book account? 
To whom do I make out the cheek .for the 
initial deposit on a passbook account? 
The interest rate query is an example of a query 
based in an exploration context, since the interest 
rate on an account affects its desirability compared 
to other accounts but has no instrumental rele- 
vance to any of the plan steps involved in opening 
an account. The query about the check payee, on 
the other hand, plays a local instrumental role in 
the open-account plan, but has no relevance out- 
side of that particular subplan. 
Some queries, of course, can arise in either kind 
of context. For example, 
What's the minimum initial deposit for a 
passbook account? 
could be either an exploration level query from 
an agent weighing the comparative advantages of 
a passbook account versus a CD, or it could be 
a domain level query from an agent who had al- 
ready decided to open a passbook account, and 
who needed to know how large a check to write to 
open it. Thus the context model for that query is 
ambiguous between the two interpretations. 
There are also whole classes of queries that may 
be generated on the plan exploration level but that 
do not arise when agents are pursuing an adopted 
domain plan, as when an agent asks queries about 
the possible plans for a goal or about possible 
fillers for a variable within a plan. For example, 
What does it take $o open an account? 
asks about the subactions in a plan being explored, 
and 
What kinds of accounts do you offer? 
asks for possible fillers of the account-class vari- 
able. Such queries imply that the agent does not 
have any fully-instantiated adopted plan in mind. 
DOMAIN PLAN REFERENCES 
Another difference between the exploration and 
domain levels comes in the patterns of references 
to domain plans. An agent pursuing an adopted 
domain plan has a single subplan in focus and typ- 
ically shifts that focus in an orderly way related 
to the sequential steps in that plan. On the other 
hand, at the exploration level, the possible pat- 
terns of movement are much less constrained, and 
conflicting alternative plans or multiple hypothet- 
ical plans may be explored simultaneously. Explo- 
ration metaplans can capture these more complex 
patterns. 
For example, agents frequently generate queries 
that compare particular features of alternative 
plans for the same goal. For instance, after ask- 
ing about the interest rate on passbook accounts, 
the agent might naturally ask about the rate for 
CD's. This kind of comparison can be modeled 
by a compare-by-feature plan exploration recta- 
plan, which represents the close discourse connec- 
tion between the similar features of the two dif- 
ferent plans. Such feature by feature comparison 
would be hard to capture in a model based directly 
on the domain plans, since the focus would have 
to jump back and forth between the two alterna- 
tives. At each step, such a model would seem to 
predict further queries about the current plan as 
more likely than a jump back to a query about 
the other plan, while at the exploration level, we 
can have plan comparison metaplans that capture 
either a plan by plan or a feature by feature ap- 
proach. 
A different kind of complex domain plan ref- 
erence can occur in a hypothetical exploration 
query, where the agent explores plans in a con- 
text that includes projected states of affairs that 
are different from her own current world model. 
Of course there is a sense in which every explo- 
ration level query is hypothetical, since it con- 
cerns the preconditions or effects of executing a 
plan to which the agent is not yet committed, but 
the issue here concerns hypothetical queries that 
assume more than the adoption of the single plan 
being explored. While modeling arbitrary hypo- 
theticals requires more than a planning system, 
there are cases where the hypothetical element in 
the agent's question can be expressed by assum- 
ing the adoption of some other plan in addition 
to the one currently explored. For example, for a 
hypothetical query like 
If I put $1000 in a 1-year CD and with- 
drew it in a month, what would be the 
40 
penalty ? 
it seems that an exploration level metaplan could 
use the purchase-CD plan to create the hypothet- 
ical context in which the query about the with- 
drawal plan penalty is to be understood. 
Thus the domain plan references in exploration 
utterances often do not correspond closely to the 
shape of the domain level plan tree. Exploration 
metaplans can supply alternative structures that 
better capture the more complex reference pat- 
terns involved in examples like feature compar- 
isons or hypothetical queries. 
MOVES BETWEEN LEVELS 
Finally, distinguishing between domain plan and 
exploration behavior is important so that the sys- 
tem can recognize when the agent moves from one 
level to the other. If an agent has been asking 
evaluative queries and then proceeds to ask a pure 
domain level query about one of those plan op- 
tions, the system should recognize that the agent 
has adopted that particular plan and is now ac- 
tively pursuing it, rather than continuing to eval- 
uate alternatives. Such an adoption of a particular 
subplan establishes the expectation that the agent 
will continue to pursue it, perhaps asking further 
domain level queries, either until it is completed 
and focus moves on to a new plan or until a plan 
blockage or second thoughts on the agent's part 
trigger a reconsideration move back to the explo- 
ration level. 
We can see the importance of this distinction 
in the care taken by human agents to make clear 
the level at which they are operating. Agents use 
mood, cue phrases, and direct informing acts to 
keep their expert advisor informed as to whether 
they are speaking of adopted plans or of ones only 
being explored, so that the expert's plan tracking 
and responses can be fully cooperative. 
STRUCTURE OF THE MODEL 
THE DOMAIN LEVEL 
The base level in the model is always a domain 
plan structure representing the plans and goals 
that the agent is believed to have adopted and 
to be currently pursuing. These plans are orga- 
nized (adapting a technique from gautz \[Kau85\]) 
into a classification hierarchy based on their ef- 
fects, so that the subplan children of a class of 
plans represent alternative possible strategies for 
achieving the given effects. The plans also include 
traditional action links outlining their action de- 
composition in terms of more primitive plans. 
There are two classes of predicates in a plan 
definition: precondition., which must be true for 
the plan to be executed but which can be re- 
cursively planned for, and constraint, (using Lit- 
man's word--Carberry calls them "applicability 
conditions") which cannot be recursively planned 
for. Each predicate is also classified as to its rel- 
evance, where internally relevant predicates are 
those whose bindings must be known in order 
to execute the plan and external predicates are 
those whose bindings are relevant when evaluating 
the plan from the outside. Thus, using the ear- 
lier examples, the payee identity for write-check 
is only internally relevant, the interest rate for 
open-savings-account is only externally relevant, 
and the minimum initial deposit feature is rele- 
vant both internally and externally. This heuris- 
tic classification of predicates is used to indicate 
which ones can be expanded at the domain vs. 
exploration levels. 
THE EXPLORATION LEVEL 
The basic exploration metaplan is explore-plan it- 
self, which takes an instantiated domain plan as 
its single parameter. The expected default ex- 
ploration pattern simply follows the domain plan 
tree shape, exploring the subplans and actions be- 
neath that plan, using the explore-subplan and 
explore-subactlon metaplans. This default behav- 
ior is compiled in by linking each exploration level 
node to the explore-plan nodes for the subplans 
and subactions of the domain plan it references. 
Thus when the system models a move to the ex- 
ploration level from a given domain plan node, the 
entire subtree of possible plans and actions be- 
neath that node is also instantiated beneath the 
initial exploration level node. 
The more complex exploration level strategies 
are encoded as metalevel subplans and subactions 
of explore-plan. For example, compare-subplans is 
a subplan of explore-plan, and compare-by-feature 
is in turn one subplan of compare-subplans. The 
system works from this library of exploration 
metaplans to create trees of possible contexts be- 
neath each explore-plan node that model these al- 
ternative strategies of plan exploration. 
THE DISCOURSE LEVEL 
The metaplan structure directly underlying an ut- 
terance is always a discourse metaplan, though it 
may be as simple as an ask-value metaplan (like 
Litman's identify-parameter) directly based in the 
current domain plan context. As in Litman's ap- 
proach, phenomena like clarification subdialogues 
41 
Discourse: 
* (ask-sub-plans (open-savings-account agent1 ?bank)) 
Exploration: 
* (explore-plan (conduct-banking-activlty agent1)) 
* (explore-plan (open-savings-account agent1 ?bank)) 
Domain: 
* (manage-money agent1) 
* (conduct-banking-activity agentl) 
Figure 1: What kinds of savings accounts do you offer? 
can be handled by further layers of discourse meta. 
plans that introduce additional structure above 
the domain plan. In this three-level model, these 
discourse layer metaplans can also be based on ex- 
ploration level plans. 
In testing for a match between a given query and 
a discourse context like ask-value, the discourse 
metaplans have access to the set of relevant pred- 
icates from the base context. In determining that 
set, the system uses the appropriate relevance cri- 
teria depending on whether the base context is at 
the domain or exploration level. There are also 
particular discourse plans, such as the ask-fillers 
plan, that require that their base context be at 
the exploration level. 
OPERATION OF THE MODEL 
For each utterance, the system begins from the 
previous context(s) and searches for a discourse 
node (based either on a domain or exploration 
node) that matches the utterance. In the follow- 
ing example, an initial domain level context is as- 
sumed, with the default top-level goal being that 
deduced from the situation of the agent entering 
a bank and approaching the receptionist, namely, 
(conduct-banklng-activity ?agent) as a subplan of 
(manage-money ?agent). 
The matching context for the initial query, 
What kind8 of savings accounts do you 
offer? 
is seen in Figure i, with asterisks used to mark 
the current focused path. No match in the as- 
sumed context is found to this particular query 
using discourse metaplans based in domain plans, 
although one can imagine other contexts in which 
this query could be a step in pursuing an adopted 
plan, as in a journalist compiling a consumer's re- 
port on various banks. But using the normal plans 
for banking customers, this query matches only on 
the exploration level, where the agent is exploring 
the plan of opening a savings account. 
Note that an exploration level match could also 
be found by assuming that the move to the explo- 
ration level occurs at open-savings-account, sug- 
gesting that the agent has adopted not just the 
plan conduct-banking-activity, but also the more 
specific plan open-savings-account. The system 
finds both matches in such cases, but heuristically 
prefers the one which makes the weakest assump- 
tions about plans and goals adopted by the agent, 
thus preferring the model where the open-savings- 
account plan is only being explored. 
Suppose the agent continues with the query 
What's the interest raze on your passbook 
account? 
This is matched by a discourse plan based on 
exploring one of the subplans of open-savings- 
account, which was the previous exploration level 
context, as seen in Figure 2 at #1. The system 
also explores the possibility of matching to a dis- 
course plan based in a domain level plan, which 
would imply the agent's adoption of the plan. 
However, the interest rate feature has only exter- 
nal relevance, and thus cannot match queries on 
the domain level. This query does finds a second 
match as the beginning of a compare-by-feature 
(at ~2), but the heuristics prefer the match that 
is closer to the previous context, while discourag- 
ing the one-legged comparison. 
The agent's next query, 
And the rate for the investment account? 
42 
Discourse: 
* (ask-value agent ?rate (open-passbook-account ...)) 
Exploration: 
* (explore-plan (conduct-banking-activity agent1)) 
* (explore-plan (open-savings-account agent1 ?bank)) 
* (explore-plan (open-passbook-account agent1 ?bank)) #1 
(compare-by-feature (open-savings-account agent1 ?bank) 
(compare-feature (open-passbook-account ...) (interest-rate-of ...)) #2 
Domes: 
* (manage-money agent1) 
* (conduct-banking-activity agent1) 
Figure 2: What's the interest rate for the passbook account? 
Discourse: 
* (ask-value agent1 ?rate (interest-rate-of ...)) 
Exploration: 
* (explore-plan (conduct-banking-activity agent1)) 
* (explore-plan (open-savings-account agent1 ?bank)) 
(explore-plan (open-passbook-account agent1 ?bank)) 
(explore-plan (open-investment-account agent1 ?bank)) #i 
* (compare-by-feature (open-savings-account ...)(interest-rate-of ...)) 
(compare-feature (open-passbook-account ...)(interest-rate-of ...)) 
* (compare-feature (open-investment-account ...)(interest-rate-of ...)) #2 
Domain: 
* (manage-money agent1) 
* (conduct-banking-activity agent 1) 
Figure 3: And the rate for the investment account? 
can also be matched in two different ways, as 
seen in Figure 3. One way (at #1) is based in 
an explore-plan for open-investment-account, sug- 
gesting that the agent has simply turned from ex- 
ploring one plan to exploring an alternative one. 
But this query also matches (at #2) as a second 
leg of the compare-by-feature subplan of explore- 
plan, where the query is part of the comparison 
between the two kinds of savings accounts based 
on the interest rate offered. Since that serves as 
a close continuation of the feature comparison in- 
terpretation of the previous query, the latter in- 
terpretation is preferred. 
The following two queries 
How big is the initial deposit for the,pass- 
book account? 
And for the investment account? 
can be matched by a sibling compare-by-feature 
subtree, as seen in Figure 4. This approach is thus 
able to represent the logical feature-by-feature 
structure of such a comparison, rather than having 
to bounce back and forth between explorations of 
the two subplan trees. 
The next query, 
OK, tuho do I see to open a passbook ac- 
count? 
makes a substantial change in the context, as 
43 
Discourse: 
* (ask-value agentl ?deposit (init-deposit-of ...)) 
Exploration: 
* (explore-plan (conduct-banking-activity agentl)) 
* (explore-plan (open-savings-account agentl ?bank)) 
(explore-plan (open-passbook-account agentl ?bank)) 
(explore-plan (open-investment-account agentl ?bank)) 
(compare-by-feature (open-savings-account ...)(interest-rate-of ...)) 
(compare-feature (open-passbook-account ...)(interest-rate-of ...)) 
(compare-feature (open-investment-account ...)(interest-rate-of ...)) 
* (compare-by-feature (open-savings-account ...)(init-deposit-of ...)) 
* (compare-feature (open-passbook-account...)(init-deposit-of ...)) 
* (compare-feature (open-investment-account ...)(init-deposit-of ...)) 
Domain: 
* (manage-money agent 1) 
* (conduct-banking-activity agentl) 
Figure 4: How big is the initial deposit for the passbook accountf And for the investment account? 
Discourse: 
* (ask-fillers agentl ?staff) 
Doma~l.* 
* (manage-money agent1) 
* (conduct-banking-activity agent1) 
* (open-savings-account agentl ?bank) 
* (open-passbook-account agent1 ?bank) 
* (fill-out-application agent1 ?staff ) 
Figure 5: OK, who do I see to open a passbook accountf 
shown in Figure 5. Since the choice of the bank 
personnel for opening an account is an internal fea- 
ture that can only be queried on the domain level, 
the only matches to this query are ones that imply 
that the agent has adopted the plan that she was 
previously exploring. Modeling that adoption, the 
parallel path in the domain tree to the path that 
was being explored becomes the current domain 
context, and the matching discourse plan is based 
there. The cue phrase "OK", of course, is a fur- 
ther signal of this change in level, though not one 
the system can yet make use of. 
In spite of that plan adoption, the agent can 
later reopen an exploration context concerning a 
subplan by saying, for example, 
What kinds of checks do you hayer 
She could also raise a query that implies a recon- 
sideration of the previous plan adoption by saying 
I forgot to ask whether there are any 
maintenance charges on this account. 
which would reestablish an exploration context of 
choosing between the passbook and investment ac- 
counts. 
COMPARISON WITH 
EXISTING WORK 
The general framework of using domain plans to 
model discourse structure is one that has been 
44 
widely pursued and shown to be fruitful for var- 
ious purposes \[All79, AP80, Car84, Car85, GS85, 
Sid85\]. Important extensions have been made 
more recently in plan classification \[Kau85\] and 
in modeling plans in terms of beliefs, so as to be 
able to handle incorrect plans \[Po186, Polg0\]. 
The most direct precursor of the model pre- 
sented here is Litman and Allen's work \[Lit85, 
LA87, LAg0\], which combines a domain plan 
model with discourse metaplans in a way that can 
model utterances arising from either the normal 
flow of domain plans, clarification subdialogues, 
or cases of domain plan modification. Like explo- 
ration metaplans, their discourse plans can handle 
examples that do not mirror the execution struc- 
ture of the domain plan. Their system, however, 
makes the assumption that the agent is pursuing 
a single domain plan. While the agent can mod- 
ify a plan, there is no way to capture an agent's 
exploration of a number of different domain plan 
possibilities, the use of varying exploration strate- 
gies, or the differences between utterances that are 
based on exploration plans vs. those based on do- 
main plans. 
Carberry developed a model \[Carg0\] that is sim- 
ilar to Litman's in combining domain plans with 
a discourse component, although this model's dis- 
course plans operate on a separate stack rather 
than as a second layer of the domain plan model. 
While the mechanisms of her model cover a wide 
variety of discourse goals, they make no distinc- 
tion between domain and exploration plans. They 
are thus also limited to following a single domain 
plan context at a time. 
In earlier work \[Ram89a\], I presented a model 
that accounts for the plan refining and query as- 
pects of plan exploration by using a tree of plan- 
building metaplans, and much of that structure is 
incorporated in this model. However, that version 
uses only a single layer of plan-building metaplans, 
so that it is strictly limited to plan exploration 
discourse. It thus cannot model queries arising di- 
rectly from the domain level, nor can it model the 
moves of plan adoption or reconsideration when 
the agent switches levels. The plan-building trees 
in that earlier version are also limited to following 
the structure of the domain plans, and so are un- 
able to represent comparison by features or other 
alternative exploration strategies, and that earlier 
model also lacks a separate discourse component. 
Lambert and Carberry \[Lamg0, LCgl\] are cur- 
rently working on a new, three-level approach that 
has much in common with the one presented here. 
One interesting difference is that the three levels 
in their model form a hierarchy, with discourse 
plans always rooted in exploration plans. While 
this may be appropriate for information-seeking 
discourse, allowing discourse plans to be rooted 
directly in domain plans can provide a natural 
way of representing utterances based directly on 
adopted plans. Overall, their model makes signifi- 
cant contributions on the discourse level, allowing 
for the recognition of a wide range of discourse 
plans like expressing surprise or warning. In con- 
trast, the main focus in this work has been on the 
exploration level, modeling alternative exploration 
strategies, and plan adoption and reconsideration. 
It would be fruitful to try to combine the two ap- 
proaches. 
IMPLEMENTATION AND 
FUTURE WORK 
The model presented here has been implemented 
in a system called Pragma (redone from the ear- 
lier Pragma system \[RamSOb\]) which handles the 
examples covered in the paper. Since the focus is 
on modeling plan exploration strategies, the initial 
context is directly input in the form of a domain 
plan with its parameter values, and the queries 
are input as meaning representations. The out- 
put after each query is the updated set of context 
models. 
The system has been exercised in the banking 
and course registration domains, though it is only 
populated with enough domain plans to serve as 
a testbed for the plan exploration strategies. The 
exploration level is the most developed, including 
metaplans for constraining or instantiating plan 
variables and for exploring or comparing subplans 
using various strategies. The discourse level cur- 
rently includes only the metaplans ask-value, ask- 
plans, and ask-fillers. 
Important next steps include expanding the col- 
lection of exploration level metaplans from the 
samples worked out so far to better character- 
ize the full range of plan exploration strategies 
that people actually use, validating that collec- 
tion against real data. It would be particularly 
interesting to add coverage for the hypothetical 
queries discussed above, where the assumed event 
is another known domain plan. The coverage of 
discourse level metaplans should be expanded, to 
better explore their interaction with exploration 
plans. The system should also be made sen- 
sitive to other indicators for recognizing moves 
between the exploration and domain levels be- 
sides the class of predicate queried, including verb 
45 
mood, cue phrases, and direct inform statements 
by the agent. 
CONCLUSIONS 
This work suggests that plan exploration meta- 
plans can be a useful and domain independent 
way of expanding the range of discourse phenom- 
ena that can be captured based on a model of the 
agent's domain plans. While the more complex 
exploration strategies complicate the plan recogni- 
tion task of connecting discourse phenomena with 
the underlying domain plans, exploration meta- 
plans can successfully model those strategies and 
also allow us to recognize the moves of plan explo- 
ration, adoption, and reconsideration. 
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