From: AAAI Technical Report SS-95-07. Compilation copyright © 1995, AAAI (www.aaai.org). All rights reserved.
The Role of Plans and Planning in
Task-Related Discourse
1990).In thesecontexts
theywillinterleave
planning
and communication
aboutthe plan. An actionrepresentation
thatcandescribe
incomplete
or incorrect
nature
of plansandtheprocess
usedto correct
or completethemisessential.
R. Michael Young
Intelligent
SystemsProgram
University
of Pittsburgh
Pittsburgh.
PA 15260
myoung+@pitt,
edu
Previous
Work: Generation
Enablement
and
Previous work in action representation ;for task-related
discourse (Pollack 1990; Balkanski 1994; DiEugenio
1993a) has focused on describing relationships that
Introduction
hold between a pair of actions. Specifically, they have
Knowledge
representation
schemesfor the automatic
dealt almost exclusively with two such relationships:
generation
of action-related
natural
language
mustacgeneration andenablement. Informally,
enablement
is
countfor the relationships
commonlydescribed
in
the
relation
that
holds
between
two
actions
when
one
naturally
occurring
text.Previousworkrepresentactionestablishes
conditions
needed
fortheotheracingactionin task-related
discourse
(Pollack
1986;
tion
to
execute.
Generation
is
the
relationship
that
DiEugenio
1993b;Balkanski
1994)has useda repreholdsbetween
twoactionswhentheactof performing
sentationbasedon Goldman’s(Goldman1970)work
oneactionis a waltof performing
theother- essenin the philosophy
of action.Theseapproaches
have
tially
the
first
action
acts
as
a
one-action
subplan
for
beensuccessful
in accounting
fortextdescribing
the
performing
the
second.
relationships
betweentwosteps,butwhenviewedin
Thefollowing
definitions
areabbreviated
versions
of
thecontext
of theplanin whichthosestepsareemthose
found
in
Di
Eugenio
(DiEugenio
1993b):
bedded,
theirrepresentation
canbe seento lackspeciDefinition
1 (Generation)
Actiona directly
generficity.
Furthermore,
itslimited
scopecannotaccount
atesaction ~ if and only if 1) a is not part of doing ~,
fora widerangeof relationships
typically
expressed
in
~) a and ~ are actions performed by the same agent at
task-related
discourse.
the same time, 3) there are a set of generation-enabling
conditions C that hold during the performance time of
In contrastto the workbasedon Goldman’sapproach,
researchers
in AI planning
haveconcentrated a and J} whenever a occurs and the conditions C hold,
I~ also occurs.
on the development
of planningalgorithms
and the
Action a indirectly generates action ~ if and onllt if
datastructures
thatmakethe production
of sound
a is an element of a partialllt ordered set of actions 8
planscomputationally
manageable.
Thereis a corthat directllt #enerates1~.
respondence
betweentheinter-step
relationships
expressed
in utterances
andthestructure
of plansproDefinition 2 (Enablement) Action a enables acducedbyrecent
workin hierarchical
andpartial-order, tion ~ if and onllt if1} the time of a is prior to the time
causallink(POCL)planning.
Thispaperdescribes
of ~, ~) there are a set of conditions, C, such. that one
work-in-progress
thatrelates
therepresentational
reof the conditions in C, ei, holds as a result of the perquirements
of task-related
natural
language
generation formance of a and either there is a third action 7 such
to recently
developed
models
of plans.
that 7 generates ~ under C; or C are the ezeeutabilitlt
conditions on i~.
Representational
Requirementsfor
These definitions have proved particularly useful
DiscourseSystems
for describing relationships between steps underlying
Several key representational issues have been identified
means clauses and purpose clauses (Balkanski 1994;
in previous work on task-related discourse. First, the
DiEugenio 1993b). Means clauses are "by" constructions of the form "Do a by doing if’ and are typically
representation language must describe the role an acused to express generation, as in "Travel to Memphis
tion willplayina particular
task;thatis,howitbears
on theotheractions
underdiscussion.
Thisisessential by taking the train.". Purpose clauses are infinitival
"to" constructions of the form "Do a to do/3." Purforexplaining
therationale
behindtheparticular
way
an actionis included
in thetask.Second,
thereprepose clauses can be used to express both generation,
as in the clause "take the train to travel to Memphis,"
sentation
language
mustdescribe
actions
andtasksin
a hierarchical
manner.
Thisis necessary
forgenerating or to express enablement, as in the clause "remove the
theabstract
descriptions
of actions
commonly
foundin
faceplate to replace the fuse."
Unfortunately, Definitions 1 and 2 suffer from two
natural
language
descriptions.
major limitations. First, while they are adequate when
Finally,
actionrepresentations
mustbe capable
of
describing
partialplans.Conversations
aboutplans
considering a pair of actions in isolation, they are not
oftenoccurin contexts
whereagentsarecollaborat- precise enough when viewing the actions in the coning aboutthe construction
of plan(Grosz& Sidner
text of the plan in which they occur. For instance,
200
when there are several prior steps a~ that assert an
executability
condition
forsomestep/~,eachc~ enables/~.
Furthermore,
enablement
holdsevenif there
areintervening
stepsthatundotheasserted
precondition.In theformercase,systems
generating
enablernenttextcannotdetermine
whichstepto use when
constructing
a purpose
clause.
In thelatercase,those
systems
mayselecta stepwhoseeffectis undonebeforethecondition
is needed,
effectively
describing
an
unsoundplan.Balkanski
(Balkanski
1994),Di Eugenio(Digugenio
1993b)andDelin,et al (Delinet
1994)encounter
whatareessentially
thesamedifficultiesduring
textinterpretation,
inthepost-hoe
analysis
of instructional
textcorpora
by humananalysts.
Furtherquestions
mustbe answered,
however,
beforeappropriate
textual
descriptions
of actioncanbe
generated
fromthisrepresentation.
Forinstance,
what
interval
musttheconditions
C holdoverwhena setof
stepsindirectly
generate
a parentstepfl?How does
thetemporal
co-occurrence
constraint
on direct
generationapplyto a setof actions
in indirect
generation?
Whattemporal
representation
is usedto relatesteps,
particularly
sub-steps
thatoccurduringtheexecution
of a parentstep?
Thesecondlimitation
of thesedefinitions
is that
theydo notaccount
fora numberof relationships
betweenactions
thataredescribed
in task-related
discourse.
Forinstance,
as Di Eugenio
pointsout(DiEugenio1993b),
naturally
occurring
discourse
oftendescribes
a relation
shecallsa mai~teaa~ce
9oal-that
is,a condition
thatmustholdovera specified
interval.
Theserelations
correspond
closely
to theprotection
intervals
usedin least-commitment
planning.
Descriptionsof additional
relations
between
plancomponents
canbe foundin task-related
discourse,
including
ordering
constraints,
variable
bindings
andthedescriptionof planflaws- threats,
openconditions
andunexpanded
abstract
steps.Noneof theserelations
are
expressible
usingthedefinitions
of generation
andenablement
givenabove.
Theselimitations
aredue,in part,to thelackof
grounding
in a formalrepresentation
of action.The
following
section
describes
an approach
towards
action
representation
expressed
in termsofactual
planstructures,
makingproperdefinition
muchmorestraightforward.
& Weld 1991; McAllister & Rosenblitt 1991) by incorporating actiondecomposition
directly
intothecausal
linkframework<
DPOCL extends the UCPOP (Penberthy & Weld
1991)algorithm
by addinga second,
hierarchical
componentto planconstruction.
Thestepsthatareadded
to a DPOCLplanaredivided
intotwoclasses.
Primitivesteps are those actions directly executable by the
agent for whomthe plan is produced; composite steps
are steps acting as abstract descriptions of the subplans that achieve the composite step’s effects. In addition to the standard POCL-style causal links that
connect steps, DPOCLplans contain decompositional
links connecting composite steps to the steps that play
a role in their subplans. Each subplan is bounded by
a null initial step whose effects are the preconditions
of the parent and a null final step whose preconditions
are the effects of the parent. A DPOCLplan is complete once all threats are resolved, all preconditions are
established and all composite steps are expanded.
There are several factors that differentiate DPOCL
plans from those produced by other hierarchical planners and make them an appropriate choice for a representation for natural language generation. First,
DPOCLplans contain the standard POCLplan structure - causal links, codesignation and ordering constraints are all available as components of the representation. As I will describe, these features are useful
when dealing with discourse used to communicateanalogous concepts. Second, unlike other hierarchical task
network (HTN) planners (Erol, Hendlcr, & Nau 1994;
Currie & Tate 1991; Sacerdoti 1977), DPOCLdoes not
replace a composite step with its subplan during decomposition. Instead, the composite step remains in
the plan, the subplan steps are added and explicitly
marked as children by decompositional links. Finally,
unlike systems that build hierarchical plans bottomup via plan parsing (Barrett & Weld 1994), DPOCL’s
control structure allows plans to be built in a topdown manner. The hierarchical
structure of DPOCL
plans and the top-down manner in which they are constructed mirrors the top-down approaches to planning
often described in discourse.
The Role of Plan Structures
Discourse
in
Plan StructuresAs A Representational
Base
Plan structures provide a language expressive enough
to account for a wide range of communication. Generation and enablcment reduce to simple relations be-
In orderto addressthe representational
limitationsdescribed
above,we are currently
developing
a formalaction-description
language(Young1994)
whosesemanticsis groundedin plan components.
The plan structures
we use are thoseproducedby
DPOCL(Young,Pollack,& Moore1994),a hierarchicalplanning
algorithm
thatextends
recentworkin
partial
order,causallink(POCL)planners
(Penberthy
1Spacelimitations require that mydiscussion here remaininformal and prevent a full description of the underlying DPOCL
plan structures or the specification of the
syntax and semantics of our plan description language. For
a full description of the DPOCL
planner, see (Young,Pollack, &Moore1994). The action description language used
to describe DPOCL
plan structures is defined in (Young
1994).
201
tween plan components. Twosteps are related by enablement precisely when they are connected by a causal
link. Similarly, a step a generates a step/3 precisely
2.
when a is connected to/3 via a decomposition link
The underlying plan representation for text describing
ordering and binding constraints is equally straightforward.
Furthermore, the plan partiality exhibited when discussion is interleaved with planning corresponds closely
to DPOCL’s(and, indeed, all POCLplanners’) incremental planning algorithm. Utterances describing how
to flesh out an incomplete plan can be interpreted in
this context as providing necessary direction for the
participants’
planning algorithm. These utterances
can then be generated by anticipating incorrect or lesspreferred planning operations and composing the specific directives to control the planner’s search.
As an example, consider the following situation.
Suppose two agents, Lou and Stu, are discussing Stu’s
plan for their making dinner together. Lou and Stu
both know that Stu is making the main dish and Lou
is makingthe dessert, but they haven’t planned things
out in any greater detail. Stu now decides to make
quiche, a dish that requires, say, six eggs. Knowing
this, Stu can determine that if Lou refines his plan
for making dessert to one making dessert by making
brownies, there will be a conflict over a critical resource, the eggs. There are several ways that Stu can
avoid this conflict by directing Lou not to form a conflicting plan. For instance, Stu can spell it all out, as
we have just done for you. Or he could say "don’t make
brownies because I’m making quiche" or even just "I’m
making brownies," counting on Lou’s own plan inference capabilities to detect and avoid the conflict.
Another critical aspect of task-related discourse is
the ability to describe the intentional relations that
hold between actions. In Bratman’s (Bratman 1987)
account of intentionality,
intention commits an agent
to action and acts as a filter on what intentions
can subsequently be adopted. Young, et al, (Young,
Moore, & Pollack 1994) provides a definition of intended actions and effects in DPOCL
plans that~has a
close correspondence to these notions
Definition 3 (Intention in DPOCLPlans)
step s is intended with respect to a plan P precisely
when s is a step in P.
.4n effect e of step si in plan P is intended precisely
when there is g causal link labeled ~oith e connectin#
si to another step sj such that either I} sj is the goal
step, ~) sj is s final step in a subplsn for step s~ and
effect e of parent step sk is intended or 3} sj has an
effect eI that is intended.
2Here we ignore the distinction between preconditions
and the generation-enabling conditions used in previous work. For a complete discussion of the differences,
see (Young1994)
202
Grosz and Kraus (Grosz & Kraus 1993) relate Bratman’s notion of intention to a description of plans held
by a group of collaborating agents. They distinguish
between intending to do an action and intending that
a condition hold in the world. Our definition of intentionality is consistent with theirs.
Just as agents are constrained in Bratman’s account
by their prior intentions, so agents using the DPOCL
planner are constrained during plan construction in the
way they can refine a partial plan without violating
its soundness. Describing a DPOCL
plan’s intentional
structure is critical, then, for constraining the ways in
which the partial plans of other collaborating agents
will be completed.
Characterizing
Communication
as
Refinement
Utterances that describe how to flesh out an incomplete plan can be generated by anticipating the points
in the planning process where another agent’s plan refinement activity is under-constrained. Similarly, these
utterances are interpreted as directives to guide the refinement of the commonplan. There are three types
of flaws that are found in DPOCLplans and that act
as motivation for cooperative plan refinement:
¯ Open Preconditions: Steps in a plan can execute
successfully only if all of their preconditions are satisfied immediately before they execute.
¯ Unexpanded abstract steps: Recall that primitive steps are those steps that are actually executable
by an agent. Abstract steps are just that - abstractions of the steps that make up their subplan. When
a plan contains an abstract step but does not contain
a subplan for that step, agents executing the plan
will be unable to determine what primitive steps to
execute for that abstraction.
¯ Threats to causal links: A causal link in a
DPOCLplan is threatened when some other step,
either a step in the commonplan or a step in the
agent’s private plan 1) asserts the negation of the
condition that labels the causal link and 2) may occur during
theinterval
spanned
by thelink.
Therearethreetypesof flaw-related
refinement
operations
thatcan be performed
on DPOCLplans.Each
addresses
onetypeof flawdescribed
above.
I. Causalllnkaddition:
Causallinksare addedto
a planto indicate
thatonestepsl is to be used
to establish
thepreviously
openprecondition
of a
second
steps2.
2.
Decompositionllnk addition"Decomposition
linksareaddedto a planwhenan abstract
stephas
no subplan.
A decomposition
operator
for the abstract
stepis chosen
andthesteps,
causal
linksand
binding
andordering
constraints
specified
in thedecomposition
operator
areaddedto theplan.Decom-
position links are then added to connect the abstract
step to those steps in its subplan.
3. Threat Resolution Three standard types of refinement operations are available to resolve a threat by
some step ss to a causal link sl ~ s2
role in (e.g., "Do a to do 9’-"). But these constraints
fail to specify the causal connection between the steps
a,/3 and precondition c, the open condition whose establishment provides the motivation for the addition
of a in the first place. "Doa to do if’ identifies the intentional connection (the enablement relation between
a and/3). In addition, this utterance implicitly describes two additional plan constraints to be incorporated into a commonplan. It describes the type of
the new step a and the ordering constraint between
a and/3. More complicated utterances, such as "Do
a to establish condition c. c must hold before/3 can
occur." make additional constraints that underlie a
purpose clause explicit.
(a) Promotion: When ss can be ordered before sl,
an ordering constraint (s3 < Sl) can be added
promote ss before sl.
(b) Demotion: When ss can be ordered after s~,
an ordering constraint (s2 < s3) can be added
demote ss after s2.
can be
(c) Separation: When binding constraints
placed on the variables in the effects of Sl or s3 so
that any conflict between the two steps is avoided,
such a binding constraint can be added to separate
the offending step.
Each utterance in the discourse serves to introduce a
set of newconstraints (e.g., newsteps, causal links and
decomposition or ordering constraints) and the process repeats until the plan is sufficiently constrained.
As I describe below, each type of constraint can be expressed using different clausal constructs; furthermore,
each type of constraint requires different types and
amounts of surrounding information to ensure proper
inclusion in the mutual plan.
New Causal Links and Steps
Whena partial commonplan has a step /3 with an
unsatisfied precondition c, a causal link must be added
to establish the needed condition. A causal link may
be created so that it originates from a step already in
the plan or so that it originates from a new step created
at the same time and specifically added to the plan to
act as a source for the causal link.
In the former case, purpose clauses can be used to
describe step overloading (Pollack 1992). That is, they
can be used to indicate that a step already in the plan
participates in more than one intentional relationship.
"Do a to do/3" indicates that, in addition to the role
that a already plays in a plan, a now contributes a
condition needed by/3.
In the later case, a new step a must be added to the
plan as a source for a new link a ~/3. The plan can
be constrained to include a by generating an utterance
such as "Do a," but the intended interpretation of this
utterance, namely that a is to be used as a source
for a particular causal link, is made difficult by the
utteranee’s ambiguity, a may contribute conditions
that could be used to establish a precondition of several
steps already in the plan.
To avoid this ambiguity, additional constraints relating a to the plan as a whole can be described in the
utterance’s surrounding clauses. Ordering constraints
relating a to steps it must follow or precede can be
mentioned, or decompositional constraints can be described that indicate which subplan, if any, a plays a
203
Decompositions
An unexpanded abstract step/3 may be decomposed in
any of several ways, each corresponding to the use of
a different decomposition operator for ffs act-type. A
particular decomposition can be identified by referring
to the steps that play a role in the generation relationship relating parent to steps in its subplan. The
means clause "Do a by doing if’ identifies this type of
decomposition.
Whena set of steps S generates/3, describing the
decomposition by using a means clause that mentions
each s in S makes for an awkward construction.
Instead, often a single, salient step is referred to in a
means clause. One can say "take the train to visit your
sister" whentaking the train is only one of a series of
steps in the subplan for visiting your sister.
What makes taking the train "salient" when describing a plan to visit your sister? Several factors in
the DPOCLsubplan could be exploited to determine
this type of salience. For instance, taking the train
could be the one step that distinguishes the decomposition operator it appears in from the other applicable
decompositions. This could be because the operator
in which it appears is the only decomposition operator
that contains a take-train step. Or it could be that
all other decompositions involve means of travel that
introduce flaws into the plan (for instance, buying an
airline ticket would leave you with too little money,
walking to your sister’s wouldleave you with too little
time, etc) and train travel does not introduce any such
flaws.
Threat Resolution
Whena mutual plan that is partial has several steps
that are unordered with respect to one another, constraints can be added to order the two components.
Ordering constraints between two steps s~ and sj arise
in DPOCLplans for one of three reasons. First, s~
might be constrained to occur before sj because a
causal link connects si to sj. Second, s~ might be constrained to occur before sj because a decomposition
operator encodes a user-defined preference. Finally, s~
might be required to precede sj due to the resolution
of a threat (see below). If s~ asserts some condition
thatthreatens
a causal
linkoutof sj,orif sjthreatensa linkintosi,theirordering
maybeconstrained
to
avoidtheconflict.
An utterance
suchas "Doa beforeif’canbe usedto
describe
newordering
constraints.
As described
above,
theseordering
constraints
arealsoexpressed
implicitly
in purpose
clauses
describing
enablement.
usedto suggest
avoiding
thethreat,
as described
in the
previous example with Stu and Lou.
As with purpose clauses, additional information can
be includedwitha negative
imperative
to makethe
intentional
connection
clear."Donotdo ~ before~,"
"Do not do c~ to do/~,"or "Do not do ~ between7
andif’allprovide
someinformation
aboutthenature
ofthethreat
thata creates.
Explanatory
Clauses
In thisviewof task-related
utterances,
explanatory
clausesareusedto makethe motivation
behindrefinements
clear.Explanatory
clausesaresubordinate
clauses
thatprovide
information
abouttheplancontextof a mainclause
describing
someplanrefinement.
Explanatory
clauses
typically
beginwithmarkers
such
as because
or since.Theseclauses
playat leastfour
roles
of intaskrelated
discourse.
Future
Work
Usinga hierarchical
planstructure
as an actiondescription
language
eliminates
theambiguity
inherent
in
previous
approaches
andextends
therepresentational
capability
of ourlanguage
to account
for thoseplan
constructs
commonly
foundin task-related
discourse.
Ourcurrent
workdefines
a formalfirst-order
language
whosetermsdenoteplanconstructs
and whoserelationsexpressgeneration,
enablement
andotherconceptsmadeexplicit
in task-related
conversation.
Often,
however,
peoplechooseto leaveimplicit
importantrelations
betweenthe stepsin plans.They
relyinstead
on theability
of theirhearer
topiecetogether
theunderlying
plans.Forinstance,
theychoose
to describe
actions
at higher
orlowerlevels
of abstraction.In addition,
theyinclude
moreorlessdetail
about
theobjects,
temporal
orderings
or particular
stepsthat
playa rolein a subplan.
And theychoosetheextent
to whichtheymakeexplicit
the tiebetweenthenew
plancomponents
andthelargerplanalready
underdiscussion.
Wheninterpreting
theseutterances,
hearers
takeintoaccount
theunspoken
relationships
between
theplancomponents
described
in a discourse
andthe
structure
of the planalreadycommunicated.
Speakers,then,anticipate
thisinferential
capacity
in order
to generate
moreeffective
communication.
TheworkthatI proposewillprovidea formalaccountof this"anticipation"
in thegeneration
of taskrelateddiscourse.
We willextendouractionrepresentation
to includea modelof thementalprocesses
undertaken
by thehearerwheninterpreting
theseambiguousor incomplete
taskdescriptions.
Ourgoalis
to construct
a system
thattakesintoaccount
theplanreasoning
capabilities
(bothplanconstruction
andplan
recognition)
of thehearerin orderto generate
more
natural
descriptions
of plan.
¯ Explanatory
clauses
canbe usedto describe
relevant
causalstructure
of theunderlying
plan.Thiscategorycoversutterances
likethe meansclausesand
purpose
clauses
described
earlier.
Forinstance,
the
italicized
testintheutterance
"takethetraintoget
to Boston."
explains
thecausalconnection
between
thetwostepsmentioned.
¯ Explanatory
clausescandescribe
thespecific
flaw
thattherefinement
proposed
in themainclauseaddresses:
- "Mopthefloorlast,sinceyoucan’twalkon the
floor
until
itdries."
"Don’t
use
bleachon the jeans,becausebleach
damages
colored
clothes."
- "Callthe tow trucknow becausewe want the
bridge
to be clearbefore
rushhour."
¯ Explanatory
clausesmay describe
how alternative
refinements
fortheflawunderconsideration
could
introduce
additional
flawsor failto refine
theflaw
at hand.Forinstance,
" Taketheplaneto Boston,
because
taking
thetrainwillgetyouintoolatethat
night."
¯ Explanatory
clausesmay describe
preferences
betweencompeting
refinements
for thecurrentflaw.
Forinstance,
"Paintthebarnbeforeyoupaintthe
flowers.
It’seasier
tofillinthelargeareas
andthen
go backandworkon thedetails."
Avoiding Threats
- Negative
Imperatives
Becausethe intentional structure of a plan must be preserved during the planning process, refinements that
create un-resolvable threats must be avoided. As Di
Eugenio describes, negative imperatives, constructs of
the form "Do not do c~," express directives to avoid
conflict (DiEugenio 1993b). Whena generation system detects that an agent may form a flawed plan with
a threatened
causallink,a negative
imperative
canbe
204
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