12 pazdziernik 2006

1
discourse and dialogue
2

• What is a unit of communication?

M hi. d4 to d6. J uhhuh. (week passes) J
a3 to a7. M hmmm. (2 weeks pass) M Queen
beats the laufer at e1. Check. ...
Theories of discourse meaning depend in part on a
specification of the basic units of a dicouse and
the relations that can hold among them.
Discourse processing requires an ability to
determine to which portions of a discourse an
individual utterance relates. Thus the role of
discourse structure in discourse processing
derives both from its role in delimiting units of
discourse meaning and...
3
What is a discourse?
Assume that you have collected an arbitrary set
of well-formed and independently interpretable
utterances, for instance, by randomly selecting
one sentence from each of the previous chapters
of this book. Do you have a discourse? Almost
certainly not. The reason is that these
utterances, when juxtaposed, will not exhibit
coherence. Consider, for example, the difference
between passages (18.71) and (18.72). (Jurafsky
and Martin695)
Consider, for example, the difference between
passages (18.71) and (18.72). Assume that you
have collected an arbitrary set of well-formed
and independently interpretable utterances,
for instance, by randomly selecting one sentence
from each of the previous chapters of this book.
Almost certainly not. Do you have a discourse?
The reason is that these utterances,
when juxtaposed, will not exhibit coherence.
vs.
Assume that you have collected an arbitrary set
of well-formed and independently interpretable
utterances, for instance, by randomly selecting
one sentence from each of the previous chapters
of this book. Do you have a discourse? Almost
certainly not . The reason is that these
utterances, when juxtaposed, will not exhibit
coherence. Consider, for example, the difference
between passages (18.71) and (18.72). (Jurafsky
and Martin695)
4

• What is a discourse?

5

• What is a discourse?
• sentences are (typically) not processed in
  isolation
• discourse, unlike an arbitratry collection of
  utterances, forms an intentionally meaningful
  whole (discourses are about something)
• discourse has structure
• segmentation and ordering
• coherence
• cohesion

6
Discourse is internally linked it hangs
together patterns of lexical connectivity ?
cohesion linguistic text-forming
devices lexical repetition, synonymy/antonymy,
ellipsis/pro-forms, enumeration, parallelism,
co-reference (anaphora)
Time flies. You cant they fly too
quickly. (Halliday and Hasan 1982)
Time flies. You cant they fly too
quickly. (Halliday and Hasan 1982)
7

• Anaphora pronominal
• My neighbor has a monster Harley 1200. They are
  huge but gas-efficient bikes.
• One should mind their own business.
• Anaphora nominal (definite NP)
• Al bought a car the other day. He took it
  out of the garage last night with the help of
  George Cottrell, and the thing gave forth such
  immense clouds of smoke that one man came running
  up and asked me where the fire was.
• I wanted a Trumpeter Swan who could play
  like Louis Armstrong, and I simply created him
  and named him Louis. The cutting of the webs
  between his toes is also fantastical, just as the
  bird itself is .

8

• Anaphora surface-count and demonstrative
• Sarah could leave but she was also given an
  option to stay she chose the latter.
• Have just driven to town, carrying our cook1 and
  our cooks dog2. Gave the one1 300 in currency
  and placed the other2 in the infirmary, with
  eczema.

9

• Temporal anaphora
• If I must declare today that I am not a
  Communist, tomorrow I shall have to testify that
  I am not a Unitarian. And the day after, that I
  never belonged to a dahlia club.
• Spatial anaphora
• The awful hot spell broke last night and today
  is clear and beautiful, Across the street,
  the entire janitorial family has blossomed out in
  pink carnations,

10

• Strained anaphora (bridging)
• John became a guitarist because he thought that
  it was a beautiful instrument.
• The house was beautiful. The door was painted
  white and the windows
• had blue shutters.

11

• Abstract entity anaphora
• Each Fall, penguins migrate to Fiji.
• Thats where they wait out the winter.
• Thats when its cold even for them.
• Thats why Im going there next month.
• It happens just before the eggs hutch.
• (Webber 1988)
• Send an engine to Elmira.
• Thats six hours.
• (Byron 2002)

12

• Ellipsis
• The well water had chemicals in it and nothing
  in the house worked as it should work.
• I Have been uncommunicative lately, and I
  have been lagging in lifes race.
• Im afraid my poem isnt as nicely written as
  Paradise Lost, but anyway, its shorter than
  Paradise Lost .
• Ultimately, even after Garcia was gone, Ruelas
  was able to cope and move on with his career.
  And indeed, he has coped and moved on with his
  career.

13
To form the intended whole discourse segments
can be connected in a limited number of ways ?
coherence there exist linguistic devices that
make structure explicit identity (sameness)
that is, that is to say, in other words,
... opposition (contrast) but, yet, however,
nevertheless, whereas, in contrast... addition
(continuation) and, too, also, furthermore,
moreover, in addition,... cause and effect
therefore, so, consequently, thus, it follows
that, ... concession (willingness to consider
the other side) admittedly, true, I
grant,... exemplification (shift from
general/abstract to specific/concrete idea) for
example, for instance, after all, an illustration
of, indeed, in fact, specifically,... discourse
comprehension consists of recognizing the
structure
14

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

15

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

16

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

17

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

18

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

19

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

20

• Coherence vs. Cohesion
• coherence structural, functional relations
  between sentences
• cohesion non-structural, text-forming relations
  that tie parts of discourse together

When Teddy Kennedy paid a courtesy call on Ronald
Reagan recently, he made only one Cabinet
suggestion. Western surveillance satellites
confirmed huge Soviet troop concentrations
virtually encircling Poland. (Hobbs 1982)
E Forks have windows. P Yes they do.
Augmented pretension. Four plus four equals
sixteen. It is a larger element, its
photographic and phototrophic, but it is a
higher number, higher course-work. It grows
through evaporation or nocturnalism, it is
sleepy, you rediscover it and I suppose
forks could have windows through evaporation.
21
Discourse modeling intentional
approach discourse participants have certain
goals (agendas) to achieve utterances actions
that realize the intentions speakers plan wrt.
communicating intentions ties the
discourse together discourse understanding
recognizing speakers intentions
22
Intentional Approach (Grosz and Sidner 86) three
dimensions of discourse linguistic structure
the utterances intentional structure hierarchy
of intentions (communicative goals) attentional
structure model of objects, properties and
relations that are salient at each point in
discourse (dynamically changing)
23
Intentional Approach (Grosz and Sidner
86) Linguistic structure discourse segments
relations that hold between them
24
Intentional Approach (Grosz and Sidner
86) Linguistic structure discourse segments
relations that hold between them (para-)
linguistic expressions reflect discourse
structure cue phrases, aspect, tense,
intonation, gesture discourse structure
constraints discourse interpretation anaphora
resolution
25
Intentional Approach (Grosz and Sidner
86) Attentional structure participants focus
of attention (what is attended to) modeled by
focus spaces objects and relations in
focus changes insertion and deletion rules
26
Intentional Approach (Grosz and Sidner
86) Intentional structure Discourse Purpose
(DP) purpose/intention held by discourse
initiator e.g. make hearer intend to
perform a task, believe a fact, believe
that one fact supports another fact, identify
an object, identify a property of an
object assumption one per discourse
27
Intentional Approach (Grosz and Sidner
86) Intentional structure Discourse Segment
Purpose (DSP) how given segment contributes to
DP
28
Intentional Approach (Grosz and Sidner
86) Intentional structure Hierarchy of
intentions dominance DSP1 dominates DSP2 if
satisfying DSP2 is intended to provide part of
satisfaction of DSP1 precedence DSP1
precedes DSP2 if DSP1 must be satisfied before
DSP2
29
Intentional Approach (Grosz and Sidner
86) Intentional structure Hierarchy of
intentions dominance DSP1 dominates DSP2 if
satisfying DSP2 is intended to provide part of
satisfaction of DSP1 precedence DSP1
precedes DSP2 if DSP1 must be satisfied before
DSP2
30

• Discourse modeling functional approach
• relations between discourse units
• relations may be made explicit by linguistic
  cues
• model domain-independent rhetorical structure
• compositionally built discourse
  tree

31

• Rhetorical Structure Theory (Mann and Thompson
  87)
• nucleus(N) vs. satellite(S) segments core
  vs. peripheral part of the message
• nuclearity principle
• relations defined in terms of
• constraints on the nucleus
• constraints on the satellite
• constraints on the comination of N and S
• effect achieved on the text receiver
• classical RST 24 relations, (Mann, 2005)
  30 relations

32

• Rhetorical Structure Theory (Mann and Thompson
  87)
• example relations
• Elaboration set/member, class/instance/whole/p
  art Contrast multinuclear Condition S
  presents precondition for N Purpose S
  presents goal of action in N Sequence
  multinuclear Result N results from something
  presented in S

33

• Rhetorical Structure Theory (Mann and Thompson
  87)
• Evidence S provides evidence for what N claims
• constraints on N Reader might not believe N to
  a degree satisfactory to Writer
• on S R believes S or will
  find it credible
• on N and S R's comprehending S
  increases R's belief of N
• effect of W R's belief of N is
  increased

34

• Rhetorical Structure Theory (Mann and Thompson
  87)
• Evidence S provides evidence for what N claims
• constraints on N Reader might not believe N to
  a degree satisfactory to Writer
• on S R believes S or will
  find it credible
• on N and S R's comprehending S
  increases R's belief of N
• effect of W R's belief of N is
  increased
• George Bush supports Big Business. N
• He is sure to veto House Bill 1711. S

35

• Rhetorical Structure Theory (Mann and Thompson
  87)
• (volitional) Cause S presents a cause that
  motivates N
• constraints on N N is a volitional action or
  else a situation that could have arisen from a
  volitional action
• on N and S S could have caused the
  agent of the volitional action in N to perform
  that action without the presentation of S, R
  might not regard the action as motivated or
  know the particular motivation N is more
  central to W's purposes than S.
• effect of W R recognizes S as a cause
  for the volitional action in N

36

• Rhetorical Structure Theory (Mann and Thompson
  87)
• (volitional) Cause S presents a cause that
  motivates N
• constraints on N N is a volitional action or
  else a situation that could have arisen from a
  volitional action
• on N and S S could have caused the
  agent of the volitional action in N to perform
  that action without the presentation of S, R
  might not regard the action as motivated or
  know the particular motivation N is more
  central to W's purposes than S.
• effect of W R recognizes S as a cause
  for the volitional action in N
• George Bush supports Big Business. S
• He is sure to veto House Bill 1711. N

37

• Problems with RST

38

• Problems with RST (cf. Moore and Pollack 92)

how many Rhetorical Relations are there? how can
we use RST in dialogue as well as monologue? how
to incorporate speakers intentions into
RST? RST does not allow for multiple relations
holding between parts of a discourse RST does
not model overall structure of the discourse
39

• Computation of discourse coherence
• grammar-basedanalogous to sentence grammar
  encode RRs as rules, parse (Polanyi)
• inference-basedproof-system encode RRs as
  axioms, prove coherence, e.g. by abduction (Hobbs
  et al.)
• plan-basedencode RRs as plan operators,
  instantiate plan given disourse goal
  (LitmanAllen)
• shallow rulesschemata/templates, lexical clues
  (Marcu)

40

• Automatic identification of rhetorical structure
• (Marcu 99 and later work)
• parser trained on a discourse treebank
• 90 hand-annotated rhetorical structure trees
• Elementary Discourse Units (EDU) linked by
  Rhetorical Relations (RR)
• parser learns to identify N and S and their RR
• mainly shallow features lexical, structural,
  Wordnet-based similarity
• discourse segmenter (to identify EDUs)
• trained to segment on hand-labeled corpus (C4.5)
• mainly shallow features 5-word POS window,
  presence of discourse markers, punctuation,
  presence/absence of particular syntactic items
• 96-8 accuracy

41

• Automatic identification of rhetorical structure
• (Marcu 99 and later work)
• evaluation of Marcus parser
• ? hierarchical structure easier to identify than
  rhetorical structure

42

• Dialog

43

• Dialog
• linguistic properties (cohesive devices)
• structure manifested in the dialog partys
  contributions
• speech-related phenomena
• pauses and fillers (uh, um, ..., like, you
  know,...)
• prosody, articulation
• disfluencies
• overlapping speech
• spontaneous vs. practical dialogs
• topic drifts vs. goal-orientedness

44

• Dialog
• dialog is made up of turns
• speaker A says sth, then speaker B, then A...
• how do speakers know when its time to
  contribute a turn?
• there are points in dialog/utterance structure
  that allow for a speaker shift ?
  Transition-Relevance Points (TRP) e.g.
  intonational phrase boundaries

45

• Dialog
• dialog is made up of turns
• speaker A says sth, then speaker B, then A...
• turn taking rules determine who is expected to
  speak next
• at each TRP of each turn
• if current speaker has selected A as next
  speaker, then A must speak next
• if current speaker does not select next
  speaker, any other speaker may take next turn
• if no one else takes next turn, the current
  speaker may take next turn

46

• Dialog
• some turns specifically select who the next
  speaker will be
• ? adjacency pairs
• regularly occuring, conventionalized sequences
• conventions introduce obligations to respond
  (and preferred responses)
• greeting greeting question answer
• complement downplayer accusation denial
• offer acceptance request grant
• set up next speaker expectations (significant
  silence dispreferred)

47

• Dialog
• entering a conversation we (typically) have a
  certain intention
• paradigmatic use of language making
  statements...
• ...BUT there are also other things we can do
  with words
• e.g. make requests, ask questions, give orders,
  make promises, give thanks, offer apologies
• aspects of the speaker's intention the act of
  saying something, what one does in saying it
  (requesting or promising) how one is trying to
  affect the audience

48

• Dialog speech acts
• certain actions we take in communication are
  designed to get our interlocutor(s) to do things
  on the basis of understanding of what we mean
• doing things with words Austin, 1962, later
  Searle, Davis ? speech acts
• utterances are multi-dimentional acts that
  affect the context in which theyare spoken

49

• Dialog speech acts
• dimensions
• locutionary act uttering something with a
  certain meaning
• illocutionary act act performed by means of
  uttering the words ? utterances conventional
  force
• perlocutionary act what is brought about as a
  result (intentionally or not) ? how hearer is
  affected convincing the hearer, persuading,
  surprising, making sad, laugh, etc.

50

• Dialog speech acts
• examples of illocutionary acts
• assertive get H to form or attend to a belief
  e.g. claim conclude
• directive get H to do sth e.g. order,
  request, beg
• commissive S commits to doing sth e.g.
  promise, plan, vow, bet
• expressive S expresses a psychological state,
  feeling twrd. H thank, apologize, hate,
  love
• declarations S changes the state of the world
  e.g. resign, fire, name, baptize,
  pronounce husband and wife

51

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• introduced by Stalnaker (1978) based on older
  family of notions common knowledge (Lewis,
  1969), mutual knowledge or belief (Schiffler,
  1972)

52

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• stock of knowledge taken for granted, i.e.
  assumed to be known both by the Speaker and the
  Hearer sum of their mutual, common or joint
  knowledge, beliefs, and suppositions
• sources of the assumptions evidence about
  social, cultural comunities people belong to,
  academic backgrounds, etc. (communal common
  ground)
• direct personal experiences (personal common
  ground)

53

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• What does it mean You and I (mutually) know
  that p?

54

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• What does it mean You and I (mutually) know
  that p?
• I know that p You know that p

55

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• What does it mean You and I (mutually) know
  that p?
• I know that p You know that p
• I know that you know that p You know that I
  know that p

56

• Dialog joint activity
• when entering a conversation, we pressupose that
  there exists certain shared knowledge ? common
  ground
• What does it mean You and I (mutually) know
  that p?
• I know that p You know that p
• I know that you know that p You know that I
  know that p
• I know that you know that I know that p You
  know that I know that you know that p
• ...ad infinitum...

57

• Dialog joint activity
• communication relies on collaboration
• Gricean Cooperative Principle principles of
  rational behaviour
• cooperatively interpret and contribute

58

• Dialog joint activity
• communication relies on collaboration
• Gricean Cooperative Principle principles of
  rational behaviour
• cooperatively interpret and contribute
• crucial establishing shared knowledge (adding
  to common ground)
• ? grounding

59

• Dialog grounding
• levels of interpretation of performed
  communicative act
• channel S executes, H attends
• signal S presents, H identifies
• proposition S signals that p, H recognizes
  that p
• intention S proposes p, H considers p

60

• Dialog grounding
• levels of interpretation of performed
  communicative act
• channel S executes, H attends
• signal S presents, H identifies
• proposition S signals that p, H recognizes
  that p
• intention S proposes p, H considers p
• the Hearer must ground or acknowledge Speakers
  utterance
• OR
• signal, at the level that satisfies the Speaker,
  that there was a problem
• in reaching common ground

61

• Dialog grounding
• levels of interpretation of performed
  communicative act
• channel S executes, H attends
• signal S presents, H identifies
• proposition S signals that p, H recognizes
  that p
• intention S proposes p, H considers p
• grounding feedback possible at all levels
• continued attention
• relevant next contribution
• acknowledgement
• demonstration (e.g. paraphrase, completion)
• display (verbatim)

62

• Dialog grounding
• levels of interpretation of performed
  communicative act
• channel S executes, H attends
• signal S presents, H identifies
• proposition S signals that p, H recognizes
  that p
• intention S proposes p, H considers p
• problems ...possible at all levels
• lack of perception
• lack of understanding
• ambiguity
• misunderstanding
• ? clarification and repair strategies

63

• Dialog grounding
• levels of interpretation of performed
  communicative act
• channel S executes, H attends
• signal S presents, H identifies
• proposition S signals that p, H recognizes
  that p
• intention S proposes p, H considers p
• S I can upgrade you to an SUV at that rate.
• H gazes appreciatively at S (continued
  attention)
• H Do you have a RAV4 available? (relevant next
  contribution)
• H ok / mhmmm / Great! (acknowledgement/backchann
  el)
• H An SUV. (demonstration/paraphrase)
• H You can upgrade me to an SUV at the same
  rate? (display/repetition)
• H I beg your pardon? (request for repair)

64
(No Transcript)
65
dialog systems

• goal-oriented conversational systems
• challenges
• need to understand
• interpretation context-dependent
• intention recognition
• anaphora resolution
• people dont talk in sentences...
• users self-revisions

66
dialog systems

• goal-oriented conversational systems
• how
• interactions in a limited domain
• prime users to adopt vocabulary the system knows
• partition interaction into manageable stages
• let the system take the initiative
  (predictability)

67
dialog systems

• example tasks
• retrieve information ? information-seeking
  dialogue
• seek to satisfy constraints ? negotiation
  dialogue
• perform action ? command-control dialog
• collaborate on solving a problem ?
  problem-solving dialog
• instruct ? tutorial/instructional dialogue

68
dialog systems

• modality type of communication channel used to
  convey or acquire information
• natural-language spoken or textual
  keyboard-based or both
• pointing devices
• graphics, drawing
• gesture
• combination of one of more of above (multi-modal
  systems)

69
dialog systems

• turn-taking and initiative strategies
• system initiative
• S Please give me your arrival city name.
• U Baltimore.
• S Please give me your departure city name.
• user initiative
• S How may I help you?
• U I want to go from Boston to Baltimore on
  November 8.
• mixed initiative
• S How may I help you?
• U I want to go to Boston.
• S What day do you want to go to Boston?

70
dialog systems
System Which date do you want to fly from
Washington to Denver?
Speech
Speech

Automatic SpeechRecognition
Response Generation
data, rules, domain reasoning
Words spoken Bill I need a flight from
Washington DC to Denver roundtrip
getDepartureDate
Action
DialogManagement
Spoken LanguageUnderstanding
Meaning
ORIGIN_CITY WASHINGTON DESTINATION_CITY
DENVER FLIGHT_TYPE ROUNDTRIP
71
dialog systems
hello Bill, how may I help you today?
speech technology recognition, synthesis
NLP grammars, parsers, generation, discourse,
pragmatics
AI reasoning, communication, planning, learning
human factors design, performance, usability
72
dialog systems

• dialog management
• control the flow of dialog
• when to say something, when to listen
  (turn-taking), when to stop
• update dialog context with current users
  input and output the next
• action in the dialog
• deal with barge-in, hang-ups
• dialog modeling
• what is the context
• what to say next
• goal achieve an application goal in an
  efficient way through a series of interaction
  with the user

73
dialog systems

• dialog models why?
• system and user work on a task
• dialog structure reflects the task structure
• BUT
• dialog need not follow the task-steps
• need for grounding

74
dialog systems

• examples of dialog models
• frame-based
• FSA
• Information State
• the choice depends on the complexity and nature
  of the task

75
dialog systems

• frame-based dialog models
• sets of precompiled templates for each data
  item needed in the dialog
• systems agenda ? fill the slots in the
  template
• system maintains initiative ?
  directed-questions (prompts)
• slots need not be filled in a particular
  sequence ? over-answering, actions triggered on
  other slots

76
dialog systems

• frame-based dialog models example of a frame
• SHOW
• FLIGHTS
• (getOrigin CITY)
• (getDate DATE) (getTime TIME)
• DEST
• (getDestination CITY)
• U Show me morning flights from Boston to SF on
  Tuesday.

77
dialog systems

• frame-based dialog models are not suited to
  dynamic complex dialogs
• single task/goal
• fixed dialog structure
• doesnt handle multiple topics/conversation
  threads

78
dialog systems

• FSA-based dialog models
• dialog modelled as a directed graph
• start 01 getName
• 02 getTransactionType
• 03 if type balance goto 10
• 03 if type deposit goto 20
• ...
• 50 ask(another transation?)
• if yes goto 02
• else stop

79
dialog systems

• Information State-based models
• dialog is not scripted
• rich representation (includes dialog context,
  obligations, dialog state)
• dialog contributions viewed as dialog moves
  (DMs)
• dialog move types similar to speech acts
  (command, wh-question, revision, etc.)
• stored in DM history ? multi-threaded
  conversations
• dialog model is a tree-structure
• new DMs attach to the tree and
• update the dialog state ? contextual
  interpretation
• allows for mixed-initiative

80
(No Transcript)
81

• When I use a word,' Humpty Dumpty said, in
  rather a scornful tone, it means just what I
  choose it to mean neither more nor less.'
• The question is,' said Alice, whether you can
  make words mean so many different things.'
• The question is,' said Humpty Dumpty, which is
  to be master that's all .

82

• References
• D. Byron. Resolving Pronominal Reference to
  Abstract Entities. Proceedings of ACL-02,
  pp.8087, 2002
• B. J. Grosz, K. Sparck-Jones, B. L.. Webber.
  Readings in Natural Language Processing, Morgan
  Kaufmann Publishers, 1986
• B. J. Grosz and C. L.. Sidner. Attention,
  intentions, and the structure of discourse.
  Computational Linguistics 12(3)175204. 1986
• M. Halliday, R. Hasan. Cohesion in English.
  Harlow Longman, 1976
• J. Hobbs. Towards an Understanding of Coherence
  in Discourse, in W. Lehnert M. Ringle (eds.),
  Strategies for Natural-Language Processing,
  Hillsdale NJ Erlbaum, 1982
• W. C. Mann and S. A. Thompson. Rhetorical
  structure theory A theory of text organization.
  Technical Report ISI/RS-87-190, USC/ISI, 1987
• D. Marcu. A decision-based approach to rhetorical
  parsing. Proceedings of ACL-99, pp. 365372, 1999
• B. L.. Webber. Discourse deixis Reference to
  discourse segments. Proceedings of ACL-88, pp.
  113123, 1988
• E.B. White. Letters of E.B. White, ed. D.L. Guth,
  Harper Row, New York, 1972 (example sentences
  with anaphora)