Teaching Natural Language Generation in an XML Framework
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Graham Wilcock 
University of Helsinki 
00014 Helsinki, Finland 
graham.wilcock@helsinki.fi 

1. Introduction

XML-based techniques for natural language generation are described by
Wilcock (2001), based on practical experience in developing an
XML-based generation component for a spoken dialogue system (Jokinen
and Wilcock, 2001). The basic approach is to construct a pipeline of
XSLT transformations corresponding to the different NLG processing
tasks. Wilcock (2002) gives a tutorial introduction to this approach
and to the basic NLG tasks. A related software demo showing NLG
integrated with XML web technology is described by Wilcock (2003).

Ongoing work aims at adapting this framework for use in teaching
natural language generation courses for language technology students.
The description here is largely taken from Wilcock (2003).

2. The Demonstration System

The demonstration system performs bilingual generation of responses,
in Finnish and English, as part of a Helsinki bus timetable enquiry
system.  The responses depend on the dialogue context and can vary
from full sentences to short elliptical phrases. The system
demonstrates only generation, without speech recognition,
language understanding or dialogue management.

2.1 Input Agenda 

The starting point is an agenda, a set of concepts marked with Topic
and NewInfo tags (Jokinen and Wilcock, 2001). A number of different
starting agendas are provided, and their contents can be changed as
desired.

<AG id="AG1" type="transcription" 
 timeline="Timeline1"> 
<Anchor id="AG1_anchor1" offset="1"/> 
<Anchor id="AG1_anchor2" offset="2"/> 
<Annotation id="AG1_ann1" type="Concept" 
 start="AG1_anchor1" end="AG1_anchor2"> 
<Feat name="name">route</Feat> 
<Feat name="status">new-info</Feat> 
<Feat name="value">81</Feat> 
</Annotation> 
<Annotation id="AG1_ann2" type="Concept" 
 start="AG1_anchor1" end="AG1_anchor2"> 
<Feat name="name">time</Feat> 
<Feat name="status">new-info</Feat> 
<Feat name="value">11:37</Feat> 
</Annotation> 
<Annotation id="AG1_ann3" type="Concept" 
 start="AG1_anchor1" end="AG1_anchor2"> 
<Feat name="name">place</Feat> 
<Feat name="constr">depart</Feat> 
<Feat name="status">topic</Feat> 
<Feat name="value">herttoniemenranta 
</Feat> 
</Annotation> 
</AG> 

Figure 1: An Agenda 

The agenda is represented as an annotation graph. Figure 1 shows an
agenda for a response following the enquiry

  When does the next bus leave from Herttoniemenranta? 

The departure-place is marked as topic, and the route-number and
departure-time are marked as new information. The response (generated
step-by-step in the next sections) will be

  Number 81 leaves from there at 11:37.

2.2 Text Planning 

In text planning, the content determination stage extracts the
concepts from the annotation graph. The discourse structuring stage
creates a text plan tree (here called a response plan) using the form
of template-based generation described by Wilcock (2001).

<ResponsePlan> 
<Message> 
<type>NumFromDepMsg</type> 
<concept info="NewInfo"> 
<type>bus-number</type> 
<value>81</value> 
</concept> 
<concept info="NewInfo"> 
<type>departure-time</type> 
<value>11:37</value> 
</concept> 
<concept info="Topic"> 
<type>departure-place</type> 
<value>herttoniemenranta</value>
</concept> 
</Message> 
</ResponsePlan> 

Figure 2: A Text Plan

Text plans are XML tree structures containing variable slots, filled
in later by the microplanning stages. In this example there is only
one message, typical in spoken dialogue responses.  In multi-paragraph
text generation there are large numbers of messages. Note that
departure-place is Topic, bus-number and time are NewInfo.

In the teaching system, tracing can be switched on so the text plan is
displayed.

2.3 Microplanning 

The microplanning stages are a sequence of XSLT transformations
(Wilcock, 2001). The text plan tree is replaced by a text
specification tree, here called a response specification.

At later stages of the pipeline, further information is added to the
tree or nodes in the tree are replaced by new nodes. In the referring
expression stage of microplanning, domain concepts are replaced with
linguistic referring expressions.

<ResponseSpec> 
<PhraseSpec> 
<head>leave</head> 
<subject> 
<head>number</head> 
<attribute>81</attribute> 
</subject> 
<adverbial> 
<advtype>from-place</advtype> 
<head>from</head> 
<object> 
<head>there</head> 
</object> 
</adverbial> 
<adverbial> 
<advtype>at-time</advtype> 
<head>at</head> 
<object> 
<attribute>11:37</attribute> 
</object> 
</adverbial> 
</PhraseSpec> 
</ResponseSpec> 

Figure 3: A Text Specification 

In Figure 3 the concepts of Figure 2 have been replaced by linguistic
specifications.  In the lexicalization stage, the <head> words are
inserted with their dependents, using a form of head-dependency
structure.

In the referring expressions stage, the departure-place concept which
was marked as Topic in Figure 2 has been pronominalized as there. If
the same departure-place concept were marked as NewInfo, it would be
realized by the actual text value of the departure-place.

In the teaching system, tracing can be switched on so the text
specification is displayed.

2.4 Realization 

The realization stage produces Java Speech Markup Language.

<jsml lang="en"> 
<div type="sent"> 
number 
<sayas class="number">81</sayas> 
leaves from there at 
<sayas class="time">11:37</sayas> 
</div> 
</jsml> 

Figure 4: Speech Markup 

The <head> words of Figure 3 provide the main content. In JSML <div
type="sent"> marks sentence boundaries, <sayas="number"> tells the
speech synthesizer that "81" should be pronounced "eighty-one" not
"eight one". The JSML is passed to the FreeTTS speech synthesizer
which produces the spoken response, in this case

  Number 81 leaves from there at 11:37.

References 

K Jokinen and G Wilcock. 2001. 
Confidence-based adaptivity in response generation 
for a spoken dialogue system.  SIGdial-2001, Aalborg, Denmark. 

G Wilcock. 2001. Pipelines, templates and transformations: XML for
Natural Language Generation. 1st NLP and XML Workshop, Tokyo.

G Wilcock. 2002. XML-based Natural Language Generation. XML Finland
2002, Helsinki.

G Wilcock. 2003. Integrating Natural Language Generation with XML
Web Technology. EACL-2003, Budapest.