UNDERSTANDING REPETITION IN NATURAL LANGUAGE 
INSTRUCTIONS - THE SEMANTICS OF EXTENT 
Sheila Rock 
Department of Artificial Intelligence,Edinburgh University* 
80 South Bridge, Edinburgh EH1 1HN, Scotland, United Kingdom 
sheilaraisb.ed.ac.uk 
Introduction 
Natural language instructions, though prevalent in 
many spheres of communication, have only recently 
begun to receive attention within computational 
linguistics\[5\]. Instructions are often accompanied 
by language intended to signal repetition of the ac- 
tion that they instruct. In order to develop a sys- 
tem that is able to understand instructions, with 
the goal of executing them, it is necessary to inves- 
tigate what is meant by various types of repetition, 
and the different ways in which repetition can be 
expressed. 
We focus on sentences that are instructing that 
some action is to be performed and that this action 
is to be performed more than once 1. There are two 
aspects to consider - scope (what part of the action 
that is instructed in the dialogue is to be repeated) 
and extent (how much repetition is to be done). 
This is illustrated by examples (1) and (2). 
Place a chunk of rhubarb into each tart. (1) 
Continue to layer in this way until all the (2) 
fruit is used. 
The repetition in (1) has scope on place a chunk 
of rhubarb (into a tart) and extent across all tarts. 
(2) has scope over layer in this way and extent until 
all the fruit used. Within this framework of scope 
and extent that I have described only informally, I 
discuss the issue of extent in more detail s . 
Karlin \[3\], presents a semantic analysis of verbal 
modifiers in the domain of cooking tasks. Much of 
this is pertinent to an examination of extent, in par- 
tieular the relation of different modifiers to the as- 
peetual category of an event (according to Moens & 
Steedman \[4\]). This has formed an important start- 
ing point for my work in understanding instructions 
for repetition. However, there are aspects where a 
different approach to Karlin's is required, and some 
of these are discussed in the rest of this paper. 
Semantics of verbal modifiers 
In analysing the semantics of verbal modifiers, 
Karlin\[3\] identifies three kinds of modifiers, which 
are themselves divided further. The primary cate- 
gorisations are 
*Thanks to Chris Mellish, Robert Dale and Graeme Ritchie 
for discussion about the ideas in this paper. 
1 This paper deals only with instructions, and uses the words 
sentence and instruction interchangeably. 
2A central theme of my thesis is that both scope and extent 
must be accounted for in a full treatment of repetition, but a 
discussion of scope is outwith the scope of this paper. 
279 
1 The number of repetitions of an action. 
2 The duration of an action. 
3 The speed of an action. 
It is clear that Karlin's first two primary cate- 
gories describe modifiers that are concerned with 
the repetition of an action 3, and these are exam- 
ined in detail in the next sections. First, though, it 
is useful to consider that with any action, we have 
a time interval, during which the action is to be 
performed - once or more than once. We can then 
characterise the extent of repetition in terms of this 
time interval. Modifiers of Karlin's category 2 tell 
us how long the time interval is, while modifiers of 
category 1 tell us how to carve up the time interval. 
One instruction may give information for both cat- 
egories, but this usually is for two different actions, 
such as 
Roast for 45 minutes, basting twice. (3) 
Number of repetitions - carving the interval 
In this category, Karlin enumerates classes of mod- 
ifier as follows: 
• cardinal count adverbials - turn the fish twice 
• frequency adverbials - turn the fish occasionally 
• plural objects - turn the pieces offish 
In the discussion of frequency adverbials, Karlin 
describes frequency as a continuous scale with grad- 
able terms, such as occasionally, often. This class 
should include explicit frequency in time units, as 
in every 5 minutes. 
Duration of an action - delimiting the interval 
Here, Karlin enumerates the following kinds of 
modifier: 
• explicit duration in time intervals - fry the fish 
for 10 minutes 
• duration given by gradable terms - fry the fish 
brie/Ty 
• duration co-extensive with the duration of an- 
other action - continue to fry the millet, stirring, 
until the water boils 
• duration characterized by a st'~te change - fry the 
fish until it is opaque 
• disjuncts of explicit durations and state changes 
- fry the fish for 5 minutes or until it becomes 
opaque 
3I will not consider the third, which contributes to "quality" 
of execution of an action, and does not pertain to extent of 
repetition. 
In this category, Karlin does distinguish be- 
tween "explicit duration" and "duration in grad- 
able terms", whereas in the 'Trequency adverbials" 
classification, there are not seperate classes for 
vague and explicit frequency (say turn the fish ev- 
ery 5 minutes and turn the fish often). To be more 
consistent, there should be one class within the cat- 
egory "number of repetitions of an action" that 
contains frequency adverbials 4, and only one class 
within the cat~gory "duration of an action" that 
contains duration in terms of time 5. In both classes 
there should be the possibility of being explicit or 
vague. It is also preferable to call Karlin's second 
category "duration of repetition of an action". The 
name "duration of an action" conflates the concept 
of the basic action and its repetition. The sepa- 
ration is pertinent to the view that repetition has 
scope and extent. 
Karlin analyses the remaining three classes in cat- 
egory 2 explicitly in the context of cooking tasks. 
In particular, the analysis is related to the view 
that all processes in the cooking domain must have 
culminations. The validity of this approach is dis- 
cussed in the next section. However, before doing 
that we examine Karlin's final class, "disjuncts of 
explicit durations and state changes". This is a 
class of instructions found mainly in the cooking 
domain. The example used by Karlin is (4). 
Steam for g minutes or until the mussels (4) 
open. 
Karlin asserts that 'the meaning of sentences in 
this category is not the same as that of logical 
disjunction'\[3, pg 64\], and claims that the mean- 
ing of the disjunction is that 'the state change 
(the mussels are open) is to be used to determine 
the duration of the action (2 minutes)' \[ibid\] (my 
parentheses) s . 
I agree that the meaning is not simply that of log- 
ical disjunction, but we need to examine the issue 
further. Data that I have collected gives evidence 
that the use of the or is not significant. There are 
many examples where a recipe book will give the 
same instruction, both with and without it. For 
example, 
... at least 10 minutes or until the flour is (5) 
well browned \[2, pg 120\] 
Bake for about g hours, until the rabbit and 
lentils are tender \[2, pgll9\] (6) 
Bake for 45 minutes or until the rabbit is (7) 
tender \[2, pgll8\] 
In all of these, we have an instruction describing 
one of the following scenariosT: 
Do some action until an expected state 
change occurs. This should take the du- (8) 
ration specified. 
4This is as Karlin's classification 
5This is different from Karlin's classification 
6Karlin sees these as metalinguistic disjunction, which I be- 
lieve is similar to part of my view. 
7I make no claims about exactly which of these scenarios is 
being described. 
280 
Do some action for a specified duration. If 
the expected state change does not occur 
during this time, then it is likely that some- (9) 
thing has gone wrong. 
What is really being given is a way to decide when 
to stop the action, and the use of two clauses pro- 
vides a way of deciding whether the stop state is 
successful or a failure. For success, if the state 
change has occured, then we will expect that the 
duration has also passed s. If the duration has 
passed but the state change has not oecured, or if 
the state change has occurred but the duration has 
not passed, we still reach the stop state, but in the 
failed mode. We then have disjunction for stopping 
(we stop if either the duration or the state change is 
true) but conjunction for success (stop and a nor- 
mal outcome is only true if both the clauses are 
true). We note that often domain knowledge will 
allow the hearer to determine whether the duration 
is given as a minimum or maximum time, and what 
the effect of failure is. The analysis presented here 
does not take the use of domain knowledge into ac- 
count, to give a more general analysis. 
From the point of view of repetition, what we 
are given is a stopping condition, that is coded in 
terms of two alternatives. Using an informal no- 
tation, what is being expressed with and without 
or respectively, are the following, which are equiv- 
alent: 
should-stop(action, t)*-- 
(difference(start, t,x), x >_ duration) V (state(q,t)) 
should-stop(action, t)*- (difference(start, t,x), 
x >_ duration) A 
should-stop(action, t),--- (state(q,t)) 
Thus (7) artd (6) can be represented as 
should-stop(bake, t)~--- 
(differenee(start, t,x), x >45-minutes ) 
V (tender(rabbit, t)) 
should-stop(bake, t),--- (difference(start, t,z), 
av >_ about.2-hours) A 
shoutd-stop(bake, O,-- (tender(rabbit-and-lentils, t)) 
Sometimes, the order of the two modifiers is 
different 9 indicating that the positioning of the 
clauses is not important. 
... until the meat is tender, about 45 min'llO~t ) 
utes \[2, pg 119\] 
... until the meat is meltingly tender --(11) 
about 30 minutes \[2, pgll9\] 
Karlin proposes that the duration modifier is 
only an approximation, and that it is the state 
change modifier that determines the truth of the 
sentence 1°. Most durations, however, in the do- 
main of cooking tasks, are approximations. Decid- 
ing whether a state change has been reached is also 
8This in fact seems closer to logical conjunction than logical 
disjunction. 
9The exchanged order is usually used without the or. 
l°The terms left disjunct and right disjunct are used by Kar- 
lin, but in sentences like (10) and (11) these are not helpful 
indicators. 
approximate. In a domain where durations and ev- 
idence of state change are less approximate (say in 
chemistry), it is not clear that it is always one of the 
clauses that is performing the role of establishing 
the truth of the sentence. 
Aspectual category and verbal modifiers 
Karlin's discussion is given in the context of the 
aspectual category of an event (according to Moens 
& Steedman \[4\]). This is useful as it gives a way of 
extracting semantic information. 
Karlin claims that points, culminations and cul- 
minated processes (but not process type events) can 
have a number of repetitions associated with them 
(category 1). An expression whose aspectual type is 
a process or culminated process can co-occur with a 
duration modifier (category 2). This second claim 
requires closer examination. 
First, Moens & Steedman say that 'culminated 
processes ... (do not combine readily) with a for- 
adverbial'. Yet for-adverbials are one of the classes 
of duration modifier ennumerated by Karlin. We 
look at two of the examples presented by Karlin. 
Stir for I minute. (12) 
Saute over high heat until moisture is evaP-(13) 
orated. 
The expressions in both of these (without their 
modifiers - that is Stir and Saute over high heat) 
are processes, but not culminated processes. An 
essential part of a culmination is that there is a 
consequent state \[4, pg 16\]. None of the exam- 
ples Karlin uses has a culminated process as the 
aspectual type of the action expressed. (13) could 
be seen as culminated processes if viewed together 
with the duration modifier (in other words, if it 
already co-occurs with a duration modifier), while 
(12) is a process, even with the modifier. Thus, the 
view of Moens & Steedman holds and is in fact use- 
ful in extracting semantic information. An until- 
clause signals a culmination, thus making a process 
into a culminated process. A for-adverbial does 
not change the aspectual type of a process. 
We now look at the assertion that 'Each process 
in the cooking domain must have a culmination ...' 
\[3, pg 62\]. This is accompanied by a claim that a 
verb may contain inherent information about the 
endpoint of the action it describes, as in 
Chop the onion. (14) 
which, according to Karlin, describes a culminated 
process whose endpoint is defined by the state of 
the onion. This seems quite feasible, even if it 
does require that some world knowledge is required. 
However, if we consider instead the example 
Stir the soup. (15) 
this does not describe any culmination, as there is 
no consequent state. Yet it is a process, as it may 
be extended in time. 
281 
Karlin's justification for the above assertion is 
that cooking tasks involve a sequence of steps 
with the goal of bringing about a state change. 
There are also instructions for preventing a state 
change though, for example stirring (to prevent 
sticking). We could argue that stirring brings us 
to the changed state "stirred". But then, if we look 
back to the Moens & Steedman analysis, where he 
climbed has no culmination, we could claim that 
this has the changed state "has climbed". This is 
not the spirit intended by the Moens & Steedman 
analysis, and it is more useful to see some actions in 
cooking as not having culminations. We can then 
examine what kinds of modifiers change aspectual 
category and in what manner, as presented above 
for the for- and until-adverbials. 
Conclusion 
The semantics of repetition in instructions is more 
clearly understood if we view repetition as having 
scope and extent. Within this framework, Karlin's 
work on the semantics of verbal modifiers provides a 
useful starting point. In particular, relating this to 
the aspectuai category of an instruction according 
to Moens & Steedman \[4\] is important. We can 
make use of Moens & Steedman's schema for the 
way aspect changes when modifiers are added to 
expressions, to extract semantic information. This 
will allow a fuller treatment of extent, for use in 
the development of a semantics for repetition that 
treats both scope and extent more completely. 

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\[4\] Moens, Marc & Steedman, Mark "Temporal 
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