AN OVERVIEW OF THE NIGEL TEXT GENERATION GRAMMAR 
William C. Mann 
USC/Information Sciences institute 
4676 Admiralty Way # 1101 
Marina del Rey, CA 90291 
Abstract 
Research on the text generation task has led to 
creation of a large systemic grammar of English, Nigel, 
which is embedded in a computer program. The 
grammar and the systemic framework have been 
extended by addition of a semantic stratum. The 
grammar generates sentences and other units under 
several kinds of experimental control. 
This paper describes augmentations of various 
precedents in the systemic framework. The emphasis 
is on developments which control the text to fulfill a 
purpose, and on characteristics which make Nigel 
relatively easy to embed in a larger experimental 
program. 
1 A Grammar for Text Generation - The Challenge 
Among the various uses for grammars, text generation at 
first seems to be relatively new. The organizing goal of text 
generation, as a research task, is to describe how texts can be 
created in fulfillment of text needs. 2 
Such a description must relate texts to needs, and so must 
contain a functional account of the use and nature of language, a 
very old goal. Computational text generation research should be 
seen as simply a particular way to pursue that goal. 
As part of a text generation research project, a grammar of 
English has been created and embodied in a computer program. 
This grammar and program, called Nigel, is intended as a 
component of a larger program called Penman. This paper 
introduces Nigel, with just enough detail about Penman to show 
Nigel's potential use in a text generation system. 
IThis research was Supported by the Air Force Office of Scientific Research 
contract NO. F49620.79-C-0181. The views and conclusions contained =n this 
document are those of the author and should not be interpreted as necessarily 
representing the Official polic=es or endorsements, either expressed or implied, of 
the Air Force Office Of S(;ientific Research of the U.S. Government. 
2A text need is the earliest recognition on the part of the speaker that the 
=mmeciiate situation is orle in which he would like to produce speech. In this report 
we will alternate freely between the terms speaker, writer and author, between 
hearer and reader, and between speech and text This is s=mpty partial 
accommodation of preva=ling jargon; no differences are intended. 
1.1 The Text Generation Task as a Stimulus for Grammar 
Design 
Text generation seeks to characterize the use of natural 
languages by developing processes (computer programs) which 
can create appropriate, fluent text on demand. A representative 
research goal would oe to create a program which could write a 
text that serves as a commentary on a game transcript, making the 
eventsof the game understandable. 3 
The guiding aims in the ongoing des=gn of the Penman text 
generation program are as follows: 
1. To learn, in a more specific way than has prewously 
been achieved, how appropriate text can be created 
in response to text needs. 
2. To identify the dominant characteristics which make a 
text appropriate for meeting its need. 
3. To develop a demonstral~le capacity to create texts 
which meet some identifiable practical class of text 
needs. 
Seeking to fill these goals, several different grammatical 
frameworks were considered. The systemic framework was 
chosen, and it has proven to be an entirely agreeable choice. 
Although it is relatively unfamiliar to many American researchers. 
it has a long history of use in work on concerns which are central 
tO text generation. It was used by Winograd in the SHRDLU 
system, and more extensively by others since \[Winograd 72. Davey 
79, McKeown 82. McDonald 80\]. A recent state of the art survey 
identifies the systemic framework as one of a small number of 
linguistic frameworks which are likely to be the basis for 
significant text generation programs in th~s decade {Mann 82a}. 
One of the principal advantages of the systemic framework 
iS its strong emphasis on "functional" explanations of 
grammatical phenomena. Each distinct kind of grammatical entity 
iS associated with an expression of what it does for the speaker. 
so that the grammar indicates not only what is possible but why it 
would be used. Another is its emphasis on principled, iustified 
descriptions of the choices which the grammar offers, i.e. all of its 
optionality. Both of these emphases support text generation 
programming significantly. For these and other reasons the 
systemic framework waS Chosen for Nigel. 
Basic references on the systemic framework include: 
\[Berry 75, Berry 77, Halliday 76a, Halliday 76b, Hudson 
3This was accomplished in work Py Anthony Davey \[Davey 79\]; \[McKeown 821 is 
a comoaraOle more recent study it} whlcR the generated text clescrioed structural 
and definitional aspects of a data base. 
79 
76, Hatliday 81, de Joia 80, Fawcett 80\]. 4 
1.2 Design Goals for the Grammar 
Three kinds of goals have guided the work of creating 
Niget. 
1.To specify in total detail how the systemic 
framework can generate syntactic units, using the 
computer as the medium of experimentation. 
2. To develop a grammar of English which is a good 
representative of the systemic framework and useful 
for demonstrating text generation on a particular task. 
3. To specify how the grammar can be regulated 
effectively by the prevailing text need in its 
generation activity. 
Nigel is intended to serve not only as a part of the Penman 
system, but also eventually as a portable generational grammar, a 
component of future research systems investigating, and 
developing text generation. 
Each of the three goals above has led to a different kind of 
activity in developing Nigel and a different kind of specification in 
the resulting program, as described below. The three design 
goals have not all been met. and the work continues. 
1. Work on the first goal, specifying the framework, is 
essentially finished (see section 2.1). The lnterlisp 
program is stable and reliable for its developers. 
2. Very substantial progress has been made on creating 
the grammar of English; although the existing 
grammar is apparently adequate for some text 
generation tasks, some additions are planned. 
3. Progress on the third goal, although gratifying, is 
seriously incomplete. We have a notation and a 
design method for relating the grammar to prevailing 
text needs, and there are worked out examples which 
illustrate the methods the demonstration ~aper in 
\[Mann 83\](see section 2.3.) 
2 A Grammar for Text Generation - The 
Design 
2.1 Overview of Nigel's Design 
The creation of the Nigel program has required 
evolutionary rather than radical revisions in systemic notation, 
largely in the direction of making well-precedented ideas more 
explicit or detailed. Systemic notation deals principally with three 
kinds of entities: 1} systems, 2) realizations of systemic choices 
(including function structures), and 3) lexical items. These three 
account for most of the notational devices, and the Nigel program 
has separate parts for each. 
4This work would not have been possible wtthout the active palliclpatlon of 
Christian MattNessen, and the participation and past contributions of Michael 
Halliday and other system=c=sts. 
Comparing the systemic functional approach to a 
structural approach such as context-free grammar, ATNs or 
transformational grammar, the differences in style (and their 
effects on the programmed result) are profound. Although it is not 
possible to compare the approaches in depth here, we note 
several differences of interest to people more familiar with 
structural approaches: 
• Systems, which are most like structural rules, do not 
specify the order of constituents. Instead they are 
used to specify sets of features to be possessed by 
the grammatical construction as a whole. 
2. The grammar typically pursues several independent 
lines of reasoning (or specification) whose results are 
then combined. This is particularly difficult to do in a 
structurally oriented grammar, which ordinarily 
expresses the state of development of a unit in terms 
of categories of constituents. 
3. In the systemic framework, all variability of the 
structure of the result, and hence all grammatical 
control, is in one kind of construct, the system. In 
other frameworks there is often variability from several 
sources: optional rules, disjunctive options within 
rules, optional constituents, order of application and 
so forth. For generation these would have to be 
coordinated by methods which lie outside of the 
grammar, but in the systemic grammar the 
coordination problem does not exist. 
2.1 .1 Systems and Gates 
Each system contains a set of alternatives• symbols called 
grammatical features. When a system is entered, exactly one 
of its grammatical features must be chosen. Each system also has 
an input expression, which encodes the conditions under which 
the system is entered 5 Outing the generation, the Dr0gram keeps 
track of the selection expression, the set of features which have 
been chosen up to that point. Based on the selection expression. 
the program invokes the realization operations which are 
associated with each feature chosen. 
In addition to the systems there are Gates. A gate can be 
thought of as an input expression which activates a particular 
grammatical feature, without choice. 6 These grammatical features 
are used just as those chosen in systems. Gates are most often 
used to perform realization in response to a collection of features. 7 
5Input expressions are BooLean expressions of features, without negation, ~.e. 
they are composed entirely of feature names, together with And. Or and 
0arentheses. (See the figures in the demonstration paper tn IMann 8.3} for 
examples.) 
6See the figure entitled Transitivity I =n \[Mann 83} for examDles and further 
discussion of the roles of gates. 
7Bach realization ot~erat=on is associated with just one feature, there are no 
realizat¢on operations which depend on more than one feature, and no rules 
corresponding to Hudson's function reah;'ation rules. The gates facihtate 
elimiqating this category of rules, with a net effect that the notation is more 
homogeneous. 
80 
2.1.2 Realization Operators 
There are three groups of realization operators: those that 
build structure (in terms of grammatical functions), those that 
constrain order, and those that associate features with 
grammatical functions. 
1. The realization operators which build structure are 
Insert, Conflate, and Expand. By repeated use of 
the structure building functions, the grammar is able 
to construct sets of function bUndles, also called 
fundles. None of them are new to the systemic 
framework. 
2. Realization operators which constrain order are 
Partition, Order, OrderAtFront and OrderAtEnd. 
Partition constrains one function (hence one fundle) 
to be realized to the left of another, but does not 
constrain them to be adjacent. Order constrains just 
as Partition does, and in addition constrains the two tO 
be realized adjacently. OrderAtFront constrains a 
function to be realized as the leftmost among the 
daughters of its mother, and OrderAtEnd 
symmetrically as rightmost. Of these, only Partition is 
new to the systemic framework. 
3. Some operators associate features with functions. 
They are Preselect, which associates a grammatical 
feature with a function (and hence with its fundle); 
Classify, which associates a lexical feature with a 
function: OutClassify, which associates a lexical 
feature with a function in a preventive way; and 
Lexify, which forces a particular lexical item to be 
used to realize a function. Of these, OutClassify and 
Lexi~ are new, taking up roles previously filled by 
Classify. OutClaasify restricts the realization of a 
function (and hence fundle) to be a lexical item which 
does not bear the named feature. This is useful for 
controlling items in exception categories (e.g. 
reflexives) in a localized, manageable way. Lexify 
allows the grammar to force selection of a particular 
item without having a special lexical feature for that 
purpose. 
In addition to these realization operators, there =s a set of 
Default Function Order Lists. These are lists of functions 
which will be ordered in particular ways by Nigel. provided that the 
functions on the lists occur in the structure, and that the 
realization operators have not already ordered those functions. A 
large proportion of the constraint of order is performed through 
the use of these lists. 
The realization operations of the systemic frameworK, 
especially those having to do with order, have not been specified 
so explicitly before. 
2.1.3 The Lexicon 
The lexicon is defined as a set of arbitrary symbols, called 
word names, such as "budten", associated wtth symbols called 
spellings, the lexical items as they appear in text. In order to 
keep Nigel simple during its early development, there is no formal 
provision for morphology or for relations between items which 
arise from the same root. 
Each word name has an associated set of lexical 
features. 
Lexify selects items by word name; Classify and 
OutClassify operate on sets of items in terms of the lexicat 
features. 
2.2 The Grammar and Lexicon of English 
Nigel's grammar is partly based on published sources, and 
is partly new. It has all been expressed in a single homogeneous 
notation, with consistent naming conventions and much care to 
avoid reusing names where identity is not intended. The grammar 
is organized as a single network, whose one entry point is used for 
generating every kind of unit. 8 
Nigers lexicon is designed for test purposes rather than for 
coverage of any particular generation task. It currently recogmzes 
130 texical features, and it has about 2000 texical items in about 
580 distinct categories (combinations of features). 
2.3 Choosers - The Grammar's Semantics 
The most novel part of Nigel is the semantics of :Re 
grammar. One of the goals identified above was to "s~ecify '~ow 
the grammar can be regulated effectively by the prevailing text 
need." Just as the grammar and the resuiting text are ooth very, 
complex, so is the text need. In fact. grammar and text complexity 
actually reflect the prior complexity of the text nee~ ',vh~c~ ~ave 
rise to the text. The grammar must respond selectwely to those 
elements of the need which are represente~ by the omt Demg 
generated at the moment. 
Except for lexical choice, all variability in Nigers generated 
result comes from variability of choice in the grammar. 
Generating an appropriate s\[ructure consists entirely in making 
the choices in each system appropriately. The semantics of the 
grammar must therefore be a semantics of cno~ces in the 
individual systems; the choices must be made in each system 
according to the appropriate elements of the prevailing need. 
In Nigel this semantic control is localized ',o the systems 
themselves. For each system, a procedure is defined ,.vh~ch can 
declare the appropriate choice in the system. When the system is 
entered, the procedure is followed to discover the appropriate 
choice. Such a procedure is called a chooser (or "choice 
expert".) The chooser is the semantic account of the system, me 
description of the circumstances under wnpch each choice is 
approoriate. 
To specify the semantics of the choices, we needed a 
notation for the choosers as procedures. This paper describes 
that notation briefly and informally. Its use is exemplified in the 
Nigel demonstration \[Mann C:x3j and developed in more detail ~n 
another report \[Mann 82b\]. 
To gain access to the details of the need. the choosers 
must in some sense ask questions about particular entities. For 
example, to decide between the grammatical features Singular 
and Plural in creating a NominalGroup. the Number chooser (the 
8At the end of 1982. N,gel contained about 220 systems, with all ot the 
necessary realizations speclfiecL tt ts thus the largest systemic grammar in a single 
notation, and possibly the largest grammar of a natural language in any of the 
functional linguJstic traditions. Nigel ~S ~rogrammed in INTEF:tLISP 
81 
chooser for the Number system, where these features are the 
options) must be able to ask whether a particular entity (already 
identified elsewhere as the entity the NominalGroup represents) is 
unitary or multiple. That knowledge resides outside of Niget, in the 
environment. 
The environment is regarded informally as being 
composed of three disjoint regions: 
1. The Knowledge Base, consisting of information 
which existed prior to the text need; 
2. The Text Plan, consisting of information which was 
created in response to the text need, but before the 
grammar was entered; 
3. The Text Services, consisting of information which 
is available on demand, without anticipation. 
Choosers must have access to a stock of symbols 
representing entities in the environment. Such symbols are called 
hubs. In the cOurse of generation, hubs are associated with 
grammatical functions; the associations are kept in a Function 
Association Table, which is used to reaccess information in the 
environment. For example, in choosing pronouns the choosers 
will ask Questions about the multiplicity of an entity which is 
associated with the THING function in the Function Associat=on 
Table. Later they may ask about the gender of the same entity. 
again accessing it through its association with THING. This use of 
grammatical functions is an extension of prewous uses. 
Consequently, relations between referring phrases and the 
concepts being referred to are captured in the Function 
Association Table. For example, the function representing the 
NominalGroup as a whole is associated with the hub whictl 
represents the thing being referred to in the environment. 
Similarly for possessive determiners, the grammatical function for 
the determiner is associated with the hub for the possessor. 
It is convenient to define choosers in such a way that they 
have the form of a tree. For any particular case, a single path of 
operations is traversed. Choosers are defined principally in terms 
of the following Operations: 
1. Ask presents an inquiry to the environment. The 
inquiry has a fixed predetermined set of possible 
responses, each corresponding to a branch of the 
path in the chooser, 
2. Identify ~resents an inquiry to the environment. The 
set of responses is open-ended. The response is put 
in the Function Association Table. associated with a 
grammatical function which is given (in addition to the 
inquiry) as a parameter tO the Identify operator. 9 
3. Choose declares a choice, 
4. CopyHub transfers an association of a hub from one 
grammatical function tO another. 1° 
9See the demonstration paper in \[Mann 8,3} for an explanation and example of 
its use 
10There are three athers whtCh have some linguistic slgnihcance: Pledge, 
TermPle~:lge, and Cho~ceError. These are necessary but do not Play a central rote, 
They are named here lust to indicate that the chooser notation ~s very s=m~le. 
Choosers obtain information about the immediate 
circumstances in which they are generating by presenting 
inquiries to the environment. Presenting inquiries, and receiving 
replies constitute the only way in which the grammar and its 
environment interact. 
An inquiry consists of an inquiry operator and a 
sequence of inquiry parameters. Each inquiry parameter is a 
grammatical function, and it represents (via the Function 
Association Table) the entities in the environment which the 
grammar is inquiring about. The operators are defined in such a 
way that they have both formal and informal modes of expression. 
Informally. each inquiry is a predefined question, in English, which 
represents the issue that the inquiry is intended to resolve for any 
chooser that uses it. Formally. the inquiry shows how systemic 
choices depend on facts about particular grammatical functions, 
and in particular restricts the account of a particular choice to be 
responsive to a well-constrained, well-identified collection of facts. 
Both the informal English form of the inquiry and the 
corresponding formal expression are regarded as parts of the 
semantic theory expressed by the choosers which use the inquiry. 
The entire collection of inquiries for a grammar ~s a definition of 
the semantic scope to which the grammar is responsive at its \[evet 
of delicacy. 
Figure 1 shows the chooser for the ProcessType system. 
whose grammat=cal feature alternatives are Relational, Mental, 
Verbal and Material. 
Notice that in the ProcessType chooser, although there are 
only four possible choices, there are five paths through the 
chooser from the starting point at the too, because Mental 
processes can be identified in two different ways: those which 
represent states of affairs and those which do not. The number of 
termination points of a chooser often exceeds the number of 
choices available. 
Table 1 shows the English forms of the Questions being 
asked in the ProceasType chooser. (A word ~n all cap.tats names 
a grammatical function which is a oarameter of the inquiry,) 
Table 1: English Forms of the tncluiry Operators for the 
ProcessType Chooser 
StaticConditionQ Does the process PROCESS represent a static 
condition or state of being? 
VerbalProcessQ Does the process PROCESS represent 
symbolic communication of a Kind which could 
have an addressee? 
MentalProoessQ Is PROCESS a process of comprehension. 
recognition, belief, perception, deduction, 
remembering, evaluation or mental reaction? 
The sequence of incluiries which the choosers present to 
the environment, together with its responses, creates a dialogue. 
The unit generated can thus be seen as being formed out of a 
negotiation between the choosers and the environment. This is a 
particularly instructive way to view the grammar and its semantics, 
since it identifies clearly what assumptions are being made and 
what dependencies there are between the unit and the 
environment's representation of the text need. (This is the kind of 
dialogue represented in the demonstration paper in \[Mann 83\].) 
82 
??(Static Condition 0 P~ 
/ \ 
• : : Matedal 
Figure 1 : The Chooser of the ProcessType system 
The grammar performs the final steps in the generation 
process. It must complete the surface form of the text, but there is 
a great deal of preparation necessary before it is appropriate for 
the grammar tO start its work. Penman's design calls for many 
kinds of activities under the umbrella of "text planning" to provide 
the necessary support. Work on Nigel is proceeding in parallel 
with other work intended to create text planning processes. 
3 The Knowledge Representation of the 
Environment 
Nigel does not presume that any particular form Of 
knowledge representation prevails in the environment. The 
conceptual content of the environment is represented in the 
Function Association Table only by single, arbitrary, 
undecomposable symbols, received from the environment; the 
interface is designed so that environmentally structured 
responses do not occur. There is thus no way for Nigel to tell 
whether the environment's representation is, for example, a form 
of predicate calculus or a frame-based notation. 
Instead, the environment must be able to respond to 
incluiries, which requires that the inquiry operators be 
~mplemented. It must be able to answer inquiries about 
multiplicity, gender, time, and so forth, by whatever means are 
appropriate to the actual environment. 
AS a result, Nigel is largely independent of the 
environment's notation. It does not need to know how to search, 
and so it is insulated from changes .in representation. We expect 
that Nigel will be transferable from one application to another with 
relatively little change, and will not embody covert knowledge 
about particular representation techniques. 
4 Nigel's Syntactic Diversity 
This section provides a set of samples of Niget's syntactic 
diversity: aJl of the sentence and clause structures in the Abstract 
of this paper are within Nigers syntactic scope. 
Following a frequent practice in systemic linguistics 
(introduced by Halliday), the grammar provides for three relatively 
independent kinds of specification of each syntactic unit: the 
Ideational or logical content, the Interpersonal content (attitudes 
and relations between the speaker and the unit generated) and the 
Textual content. Provisions for textual control are well elaborated, 
and so contribute significantly to Nigel's ability to control the flow 
of the reader's attention and fit sentences into larger un=ts of text. 
5 Uses for Nigel 
The activity of defining Nigel, especially its semantic parts. 
is productive in its own right, since it creates interesting 
descriotions and proposals about the nature of English and ti~e 
meaning of syntactic alternatives, as well as new notaticnal 
devices, t~ But given Niget as a program, contaimng a full 
complement of choosers, inquiry operators and related entities, 
new possibilities for investigation also arise. 
Nigel provides the first substantial opportunity to test 
systemic grammars to find out whether they produce unintended 
combinations of functions, structures or uses of lex~cal items. 
Similarly, it can test for contradictions. Again. Nigel provides the 
first substantial opportunity for such a test. And such a test is 
necessary, since there appears to be a natural tendency to write 
grammars with excessive homogeneity, not allowing for possible 
exception cases. A systemic functional account can also be 
111t tS our intention eventually to make Nigel avaJlal~le for teaching, research, 
development and computational application 
83 
tested in Niget by attempting to replicate part=cular natural texts--a 
very revealing kind of experimentation. Since Nigel provides a 
consistent notation and has been tested extensively, it also has 
some advantages for educational and linguistic research uses. 
On another scale, the whole project can be regarded as a 
single experiment, a test of the functionalism of the systemic 
framework, and of its identification of the functions of English. 
In artificial intelligence, there is a need for priorities and 
guidance in the design of new knowledge representation 
notations. The inquiry operators of Nigel are a particularly 
interesting proposal as a set of distinctions already embodied in a 
mature, evolved knowledge notation, English, and encodable in 
other knowledge notations as well. To take just a few examples 
among many, the inquiry operators suggest that a notation for 
knowledge should be able to represent objects and actions, and 
should be able to distinguish between definite existence, 
hypothetical existence, conjectural existence and non.existence 
of actions, These are presently rather high expectations for 
artificial intelligence knowledge representations. 
6 Summary 
As part of an effort to define a text generation process, a 
programmed systemic grammar called Nigel has been created. 
Systemic notation, a grammar of English, a semantic notation 
which extends systemic notation, and a semantics for English are 
all included as distinct parts of Nigel. When Nigel has been 
completed it will be useful as a research tool in artificial 
intelligence and linguistics, and as a component in systems which 
generate text. 
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84 
