Asymmetry in Parsing and Generating with Unification Grammars: 
Case Studies From ELU 
Graham Russell,* Susan Warwick,* and John Carroll? 
* ISSCO, 54 rte. des Acacias ? Cambridge University Computer Laboratory 
1227 Geneva, Switzerland New Museums Site, Pembroke Street 
russell@divmm.onige.ch Cambridge CB2 3QG 
Abstract 
Recent developments in generation algorithms have 
enabled work in nnificafion-based computational 
linguistics to approach more closely the ideal of 
grammars as declarative statements of linguistic 
facts, neutral between analysis an0_ synthesis, x-"~-oui 
this perspective, however, the situation is still far 
from perfect; all known methods of generation 
impose constraints on the grammars they assume. 
We briefly consider a number of proposals for generation, outlining 
their consequences for the 
form of grammacs, and then report on experience 
arising from the addition of a generator to an exist- 
ing unification environment. The algorithm in 
question (based on that of Shieber et al. (1989)), 
though among the most permissive currently avail- 
able, excludes certain classes of parsable analyses. 
1. Introduction 
Parsing and generation me both concerned with the 
relation between texts and representations, and in 
so far as a grammar defines this relation without 
reference to direction, it may be regarded as rever- 
sible. Yet, in practice, the program which 'applies' 
a grammar for the purpose of parsing is quite dis- 
tinct from the one which performs generation.t 
The essential difference between parsing and 
generating lies in the nature of the input. The text, 
as a string of words, traditionally establishes the 
starting point of parsing; whether the processing is 
top-down or bottom-up, the basis for selecting 
grammar roles is information associated with words 
in the lexicon. In the case of generation, there is in 
general no guarantee that the constituents of an 
input representation correspond to words; a portion 
of the input may be related directly to a given word, 
or it may be the result of combining representations 
associated to some sequence of rules, portions of 
which me ultimately related to lexical items. For 
example, if the sentence John kicked the bucket 
receives the semantic representation die(John), it is 
i Parsing and generation need not employ dif- 
ferent algorithms or control strategies; see Shieber 
(1988) for discussion. However, a truly reversible 
gram would be an entirely different undergoking 
what is described here. One such project is 
currently under way at New Mexico State Universi- 
ty (Yorick Wilk~, p.c.). 205 
relatively easy to see how during parsing the recog- 
uilion of kicked and the bucket will provide the 
necessary information (from the lexical entry for 
kick) to build that represemo~on. "l'ne representa- 
tion and the lexical items are in general related not 
dhectly, but rather via intennediate syntactic rules, 
any of which is able to manipulate the representa- 
tion in arbitrary ways; in generation, it is not possi- 
ble to identify the correct lexical item without con- 
sidering the syntactic rules which may intervene. 
The generation problem, then, consists in how 
to build a syntactic structure faom an initial 
representation, taking it as the root, and extending 
the structure 'downward' to the lexicon by select- 
ing rules from the grammar and attaching them at 
the appropriate points. 
Though unification based systems have been in 
use for parsing for a number of years, generation 
has until recently not attracted comparable atten- 
tion; Wedet-lnd (1988), Dymetmaun & lsabelle 
(1988) and Shieber (1988) describe tluee systems 
of note. Not surprisingly, given the relative infancy 
of these explorations, none of these systems is 
without problems. The most permissive of the 
current proposals appears to be Shieber et al.'s 
(1989) revision of the Shieber (1988) algorithm, yet 
several plausible grammatical analyses handled by 
the parser me beyond the capacity of even 
approach. 
This paper reports on experience arising from 
the addition of a generator component to the FLU 2 
environment; the algorithm is a variant of that pro- 
posed in Shieber et al. (1989). We first consider 
general aspects of adapting unification grammars 
initially developed for parsing to their use in gen- 
eration. A brief description of the generator in ELU 
highlights the differences and improvements we 
have adopted. We then demonstrate shortcomings 
2 "Environnement Linguistique d'Unification". 
Cf. Johnson & Rosner (1989) for a description of 
UD (Unification Device) which includes the parser 
and facilities such as procedural abswactions and 
extended data types (lists and trees) and Estival et 
al. (1989) for a description of the extended ELU 
system which incorporates the ori~ml UD plus a 
generation and translation component. 
of this class of generation algorithms on the basis of 
two case studies. 
2. Generating with Unification Gram- 
mars 
The goal of employing a single, minimally aug- 
mented, grammar for both parsing and generation 
has become more accessible with the introduction 
of declaratve grammar formalisms (cf. Kay, 1985). 
In the context of machine translation, for which the 
ELU system has been developed, the use of the 
same grammar for both tasks is highly desirable; 
indeed much of the work on bidirectional grammars 
has been carried out in centres working on MT (cf. 
Busemann, 1987; van Nonrd, to appear;, Dymet- 
mann & Isabelle, 1988; and Wedekind, 1988). 
Regardless of the application, however, the ability 
to generate with a grammar is extremely useful as a 
method of checking its adequacy. 
Despite the objective of reversibility, all of the 
systems mentioned here impose generation-specific 
restrictions on their grammars, either by limiting 
the form of possible rules or by augmenting them 
with annotations. DymeUnann & Isabelle (1988) 
require the grammar writer to specify for each role 
the order in which daughters should be generated; 
however, an order that might be correct when gen- 
erating from one structure can lead to non- 
terminating search with another. Busemann (1987) 
and Saim-Dizier (1989) describe methods of gen- 
eration which rely on the parsing of a control struc- 
ture using a specialized grammar to build the syn- 
tax of a sentence; it is questionable to what extent 
the latter two systems can be considered to operate 
with bidirectional grammars. 
Constraints imposed by Wedekind (1988) and 
van Noord (to appear) exclude certain linguistic 
analyses from generation. In order tO overcome the 
high degree of non-determinism inherent in the 
top-down approach, Wedekind stipulates that a 
daughter of a rule must be 'connected' (i.e. that its 
semantics must be instantiated) before it can be 
generated from. Less restrictively, van Noord 
stipulates similar constraints on rules, i.e. that if the 
semantics of the mother node is known, then the 
semantics of the head daughter is instantiated, and 
additionally that if the syntax of the semantic head 
is known, then the semantics of each daughter is 
known. These restrictions limit the class of possi- 
ble analyses, excluding accounts appropriate to 
LFG (Kaplan and Bresnan, 1982), HPSG (Pollard 
& Sag, 1987) and UCG (7_eevat et al., 1987). 
The disparate state of progress in parsing and 
generation raises important issues concerning the 
adequacy of grammatical descriptions and the com- 
putational tools that interpret them. A situation 
exists in which a grammar may be 'correct' for 
analysis, but 'incorrect' for generation. 
Significantly, this may be the case even when the 
restrictions and annotations mentioned above are 
taken into account. Grammatical analyses 
developed in a purely parsing environment cannot 
206 
always be transferred slraightforwardly into a for- 
mat suitable for generation. Two types of conclu- 
sion may be drawn from this: failures may be 
ascribed to inadequacies of current generator tech- 
nology, or the grammatical analyses in question 
may be re-evaluated. Practical remedies will 
involve two related strands of research; improving 
methods of generation so as to IDinimiTe restric- 
tions on the form of grammars that can be gen- 
erated fzom, and identifying problematic properties 
of grammars. It is the second of these which the 
present paper chiefly addresses, though we also 
remark, in the next section, on some enhancements 
to the Shieber et al. (1989) algorithm that have been 
incorporated in the ELU generator. 
3. The Generator in ELU 
In this section we describe the generation algorithm 
in ELU, and discuss in what respects it differs from 
that described by Shieber et al. (1989). 3 Two 
notions central to this method of generation are that 
of the 'pivot', and that of partitioning the grammar 
intO 'chaining' and 'noD-chaining ' rules. Loosely, 
the 'pivot' of a structure to be generated from is the 
lowest node in a path down semantic heads of rules 
at which the semantics of the current generation 
root structure remain~ unchanged. A ch~inlng lille 
is one in which the semantics of the object associ- 
ated with the right-hand side category that has been 
declared as the head unifies with that of the left- 
hand side category. Other rules are non-chaining 
roles. Rules that apply between the root and the 
pivot are, by definition, chaining rules; further, any 
rule which can be attached below the pivot is, by 
definition, a non-chaining rule. Rules are parti- 
tioned into these two groups drain 8 grammar com- 
pilaton. 
Once the chaining rules have been identifed, the 
grammar compiler computes the possible sequences 
of such rules alon 8 a path through their mothers and 
semantic heads. The result is a 'teachability table', 
each of whose elements is a pair of restrictor value 
sets4 representing classes of FSs which can occur at 
the top and bouom of such a path; in each case, the 
'bottom' restrictor set characterizes a pivot. A res- 
trictor set is also computed for each lexical stem, in 
order to retrieve words efficiently during genera- 
tion 
The generation algorithm uses the distinction 
between chaining and non-chaining rules as well as 
3 Our discussion will therefore assume familiari- 
ty with this paper. 
4 Restrictors are attributes selected by the writer 
of a grammar as being maximally distinctive; when 
two FSs are to be unified, their respective restrictor 
values axe first checked for compatibility, so as to 
eliminate the cost of an attempted nnificaton which 
is bound to fail. See Shieber (1985). 
that between head and non-head dauglzers, the 
reachability table for chaining rules, the semantic 
portion of the FS to be generated fi~m 5, and the 
restrictors for lexicon stems. The algorithm is: 
1. Take all grammar rules declared as 'initial' (or 
all rules in the grammar if no such declaration 
has been made); for each of these rules whose 
mother unifies with the input FS, apply the role 
top-down, building FSs for each of the 
daughters, and, starting with the head daughter, 
execute step 2 for each one. If generation firom 
the daughters is successful, compute all possible 
word-forms (as constrained by the locally avail- 
able syntactic information) for each lexical stem 
generated. 
2. Create a pivot COnSisting of just the semantic 
portion of the current FS. Non-determiniejc- 
ally perform steps 2a and 2b: 
a. Fmd a lexical stem which unifies with the 
pivot, making sure Coy checking with the 
reachability table) that the FS resulting from 
the unification can be linked through seman- 
tic heads of just chaining rules up to the 
current FS. 
b. Fmd a non-chaining rule which can have the 
pivot as mother, similarly making sure that 
the FS resulting from the unification of the 
pivot and the mother can be linked up to the 
current FS. Recursively (through 2) gen- 
erate the rule's daughters, starting with the 
head daughter. 
3. Link the pivot up to the current FS through 
semantic beads of just chaining rules (at each 
stage, before adding a new rule in the chain, 
checking with the teachability table that further 
linking will be possible) and then recursively 
(through 2) generate the non-bead Os, ghters of 
these rules. 
In this algorithm non-cbaining roles are used top- 
down, while chaining rules are used bottom-up. 
Linking information is used both to check the appli- 
cability of a lexical stem or a non-chainlng role 
when generating top-down from a pivot, and also to 
control search when generating bottom-up, by 
ensuring that the left-hand side of any role con- 
sidered still lies on a possible path through chaining 
rules to the current FS. 
One innovation of the ELU generator is that the 
notion 'semantic bead' is interpreted rather dif- 
ferently; whereas the earlier work simply defines 
the semantic bead of a rule as the daughter whose 
semantics unifies with that of the left-hand side, and 
thus leaves the notion undefined for non-chalnlng 
rules, that described here permits the grammar 
writer to identify one daughter in each rule as the 
5 The relevant paths being determined by the 
user's declaration 
semantic head. A role in which a O~ghter sluues 
the semantics of the mother can thus be made into a 
chaining rule or a non-chaining rule, according to 
whether that daughter is identified as the semantic 
head, and a rule that would otherwise have multiple 
semantic heads can be assigned just one. 6 A rule in 
which there is no such daughter will remain a non- 
chaining rule, but may nevertheless be annotated 
with a similar specification. The rationale is two- 
fold: the ability to coerce what would otherwise be 
a chaining rule to a non-chaining rule grallts the 
grammar writer more control over generation, and 
the ability to specify one daughter as semantic~dly 
more si£nlf~mnt than the others may be exploited in 
order to direct the attention of the generator 
towards !hat daughter. 
A second difference is the order of events in 
bottom-up generation. Instead of generating firom 
the non-head daughters of each chaining rule as it is 
attached, the pivot is firm linked to the root, so that, 
if backtracking is forced, effort will not have been 
spent on processing StrU~h-e that must be dis- 
carded. 
Finally, on each occasion that top-down genera- 
tion is initiated, an auempt is made to add a lexical 
item below the current root, rather than extending 
the path by application of non-chainlng rules until 
no such rule is applicable. Here, the motivation is 
that lexical information may be made available as 
soon as possible without forcing the grammar 
writer to adopt analyses that will produce bottom- 
up generation. This is important because global 
syntactic properties of a sentence are ofteu deter- 
mined by lexical information. 
4. Grammars for Generation 
4.1. Introduction 
In this section we examine more closely interac- 
tions between generator and grammar. These fall 
under two headings: (i) the presence of now 
deterwini.~m in the grammar, and (ii) the role of 
lexicalism. 
One aspect of non-detetmini.qm in generation, 
that of the ordering of role application, is partially 
overcome in FLU by the user specification of the 
bead daughter. Non-determinism with respect to 
the order of solving constraint equations is less well 
understood. The use of restrictors helps to reduce 
the number of feature structures to be considered. 
6 Thus circumventing a problem noted by 
Shieber et al. (1989, f~4) in connection with such 
rules. Van Noord (p.c.) stipulates that any daughter 
which has the same semantics as the mother, but is 
not the semantic bead, may not branch: this con- 
straint is clearly too strong, precluding, among oth- 
er things, linguistically motivated accounts of coor- 
dination. 
207 
However, in FLU, the use of relational abstractions 
as a generalization of temj~late facilities increases 
the problem considerably/Relational abstractions 
permit the grammar writer to augment the phrase 
structure rules with statements which may receive 
multiple definitions in terms of constraint equa- 
tions; the 'Linear Precedence' definition in (2) 
below is an example. This facility is a standard 
ELU device for collapsing what would in an unex- 
tended PATR-like formalLqr¢ he several distinct 
rules, thereby capturing linguistic generalizations 
that would otherwise go unexpressed. 
It is particularly impoRant to control non- 
determinism in generation, since, at least when pro- 
cessing is initiated, there is relatively little informa- 
tion available to direct the search. Expanding multi- 
ple definitions as they are encountered would give 
rise to an n~cceptable number of alternatives, 
many of which might he identical, and often the 
information from the abstraction is not required 
until all but one of the alternatives have been 
excluded by other factors. This is not always the 
case, however, and when exceptions occur their 
effect may be drastic. We now describe one such 
exception to demonstrate how an elegant analysis 
for parsing is unsuitable for generation. 
4.2. A grammar for French clitics 
A common technique in modem lexically-oriented 
grammars, and one which reflects and extends the 
traditional notion of 'valency', is to encode infor- 
marion about the various phrases with which a verb 
combines in items on a subcategorization list. The 
grammar then enforces a match between a member 
of the list and a phrase which is to combine with 
some projection of the verb and removes the item 
from the list. When a sentence is complete, i.e. the 
verb has 'found' all necessary phrases, a grammar 
may require that the list he empty, or perhaps that 
any remaining item is in some way specified as 
optional. See e.g. Shieber (1986) and Pollard and 
Sag (1987) for applications of this method. 
A complete grammar of French must account 
for the position and ordering of clitic pronouns. 
These precede the verb, while other complement 
phrases follow. Moreover, they appear in a fixed 
order, as shown in (1): 
(1) me le lui y en 
te la leur 
se les 
nous 
vons 
Up to three clitics may occur, but for the sake of 
this discussion, we consider only the simpler case 
7 Cf. Johnson & Rosuer (1989) for a fuller 
description of relational abstractions. 
of two critics as complement phrases to the verb. s 
There are of course many ways of accounting for 
their distribution; 9 the subcategorization list device 
seems a natural solution, since any complement 
phrase may be realized as a critic. The grammar 
rule in (2) introduces up to two clitics before the 
verb, their relative order determined by a relational 
abstraction which is defined by a number of 
clauses, each clause licensing one of the possible 
clitic sequences. 
(2) vplus -> CI1 C12 I-IV 
H'recede(Cll,O_2) 
List = <HV subcat> -- CII 
<vplus subcat> = List -- C12 
Precede(X,Y) 
<X person> = first/second 
<Y person> -- third 
Ptecede(X,Y) 
<X case> = accusative 
<Y case> = dative 
Some remarks on notation will be helpful: calls to 
relational abstractions are indicated by the exclama- 
tion mark, feature-value disjunction is indicated by 
the slash, and an equation of the form 
'X = Y--Z', where X and Y are lists, nnifies X 
non-detenninistically with the result of extracting 
one instance of Z from Y. 
The effect of this rule, then, is to associate a 
pair of clitics with a verb, checkln~ that they are 
correctly ordered, and unifying the subcategoriza- 
tion list of the left-hand side category with a copy 
of that of the head verb from which objects unify- 
ing with each of the clitlcs have been removed. 
The problem emerges when information 
assumed to he held in the subcategorizafion list of 
'vplus' is required in order to control further gen- 
eration. For example, if 'vplus' appears as sister to 
another complement phrase, and the same pro- 
cedure of unifying the latter with an item on the list 
takes place, then because the generator has 
suspended expansion of non-determini.~tic abstrac- 
lions, the subcategorization list itself will he unin- 
stantiated, and therefore no information regarding 
the semantics of the complement phrase will he 
available to restrict top-down generation. 
s This is something of an oversimplification, as 
not only complement phrases, but also adverbials 
and parts of complement phrases are realized as cli- 
tics. See Grimshaw (1982) for a partial LFG ac- 
count of these phenomena. We also ignore the is- 
sue of negation, which considerably complicates 
the clitic-aux-verb structure. 
9 The categorial treatment proposed in Baschung 
et al. (1987) not only makes use of order of argu- 
ments, but also codes each clitic for all possible 
combinations. 
208 
Modifications to the syntactic constituency 
assumed bere do not affect the principle; as long as 
the instanfiation of so central an element of the 
grammar as the subcategorization list is delayed, 
the problem will remain. An alternative type of 
analysis would remove the non-determinism from 
the grammar by factoring it out into a larger 
nomber of rules. This solution is not without its 
own disadvantages; the number of distinct rules 
needed by a full treatment of French critics, 
integrated with the placement of the various nega- 
tive panicles and auxiliaries, should not be underes- 
timated. We postpone further discussion of non- 
determini.~m and delay until the conclusion and turn 
now to the problem of empty semantic heads, an 
important problem for bead-driven generation algo- 
rithms. 1o 
4.3. Empty Semantic Heads 
In German and Dutch, there are two positions in a 
sentence where tensed verbs may appear: in second 
position of a main clause, and in final position of a 
subordinate clause. Once again, a multitude of ana- 
lyses are possible within ELU grammars. One 
approach is to control the distribution of verbs with 
grammar rules specific to clause-type; this solution 
gives rise to what might be felt to be an unaccept- 
able degree of duplication in the grammar. A more 
elegant approach, successful for parsing, exploits 
the possibility of assoc/ating a word or phrase 
appearing in one position within a sentence with a 
'gap' elsewbere. 
The latter analysis will be recognized as a vari- 
ant of a standard Govermnent-Binding treatment, in 
which a tensed verb in a main clause is 'raised' 
from an 'underlying' sentence-final position to a 
'surface' second position (see e.g. Haider (1985), 
Platzack (1985) for discussion of this class of ana- 
lyses). The dependency may be implemented by 
the use of a feature, say 'v2', whose value in a 
verb-second construction is a feature structure 
representing the verb to be raised, and in other con- 
stmctions an atomic constant such as 'none', which 
serves to block the dependency. At the extraction 
site, any value of 'v2' other than 'none' may be 
cashed out as an empty production. Information 
regarding the various syntactic properties of the 
raised verb is passed in the normal fashion between 
the verb's true position and the extraction site, 
wbere it is able to exert the same constraints upon 
complement phrases that a lexically-realiTed verb 
would. 
The simplified rule set given in (3) will serve as 
a basis for discussion. Recall that the generator 
operates by partitioning the rules of the grammar 
1o This problem is alluded to in Shieber et al. 
(1989, fn.4) and is discussed in a draft of an ex- 
panded version of the paper. 
209 
into classes to be applied top-down (non-ch~inlng 
rules - here 'S-gap' and 'V2') and bottom-up 
(chaining rules - here 'TOP', 'S' and 'V'). 
Bottom-up generation is only practical if the input 
structure to that phase of generation contains 
sufficient information, e.g. the verb with its sub- 
categorization list. 
(3) # Rule TOP 
TOP -> XP I-I_S 
<* cat> = top <* head> = <H_S head> 
<XP cut> = np <H_S subcat> = \[XP\] 
<H_S cat> = sbar 
#Rule V2 
Sbar -> H_V2 S 
<* cat> = sbar <H_V2 cat> - v 
<S cat> = s <* subcat> = <S suboat> 
<S v2> = H_V2 <* bead> = <S bead> 
<H_V2 head syn vfonn> = finite 
#Rule S 
S -> XPH_S 
<S cat>ffis <XP cat> = ap 
<H_S cat> = s <* v2> = <H_S v2> 
<* subcat> = <H_S subcat> -- XP 
<* head> = <I-IS head> 
#Rule V 
S -> H_V 
<S cat> =s <* head> = <H_V bead> 
<H_V cat> = v <* subcat> = <H_V subcat> 
# Rule S-gap 
S->- 
<S cat> = s <S bead> = <V2 head> 
<S v2> ffi V2 <S subcat> = <V2 subcat> 
The verb-raising analysis sketched here has the 
unfortunate property of supplying the generator 
with a semantic bead (the verb gap) about which 
nothing is known. At the stage when top-down 
processing has identified the verb gap as the start- 
ing point fog boUom-up generation, the input 
featm'e structure is underspecified. In particular, 
the subeategorization list of the missing verb is 
-ninstalltiated, and in the grammar in question, it is 
the length of this list which controls invocation of 
the recumive role 'S'. No bindings can be found, 
and the generator suspends evaluation of that equa- 
tion in the hope, in-founded on this occasion, that 
information not yet present will later allow its solu- 
tion. The result is that'S' is repeatedly added 
above 'S-gap', in a non-termlnating attempt to 
ensure completeness of the search. 
Van Noord (1989) describes two solutions to 
this problem, both of which are additions to the ori- 
ginal program, and whose only motivation (so far) 
is to overcome this specific problem. The first, 
somewhat ad-hoc, solution allows the verb to have 
as one of its morphological realizations the empty 
string. Since word forms are generated at the end 
of processing by a morphological front-end, the 
generator can posit the same word in both positions 
(for the purpose of relrieving its subcategorizafion 
behaviour f~om the lexicon, for example). The 
morphological component then generates one 
empty string and one full word according to the 
position of the verb (i.e. in a main or subordinate 
clause). "\['nis mechani.~n is not available in ELU. 
The second solution adds an additional 'connect' 
clause in the Prolog program, specific to gaps, in 
order to assure that the gap is first instanfiated 
before further processing; this solution raises the 
issue of I~ming programs to treat specific problems 
as they are encountenxL 
There are other constructions which raise the 
same kind of problem; the fronting of apparently 
non-constitnent verbal sequences in German (Ner- 
boone, 1986) introduces more complex dependen- 
cies, while in English the phenomena of Gapping 
and Verb-Phrase Ellipsis both manifest themselves 
syntactically in the absence from a sentence of a 
verb and possibly other material. Here, the 
difficulty is, if anything, greater, as the dependen- 
cies in question are anaphoric in nature, rather than 
syntactic. 
5. Conclusion 
We have seen, in the preceding section, how in 
order to write grammars suitable for use with the 
generator, one must either modify the technical 
aspects of the grammar or dispense with cemfin 
classes of grammatical analysis (losing the benefits 
of relational abstraction on one hand, and lexical- 
ism on the other, for example). Both of these may 
be interpreted as restricting the freedom of the 
grammar writer. The problematic case illustrated in 
section 4.2 raises the issue of non-deterrolni~m, a 
potential pitfall for all unification-based systems. 
In parsing, the result may be long processing limes, 
but when generating with algorithms of this class, 
the consequence is often non-tern~inafion. As 
Shieber et al. (1989, fn.4) observe, failure to choose 
the right daughter as the starting point for recursive 
generation may prevent tenuinafion. 
The desire to exploit the power of unification by 
using the lexicon as a repository of essentially syn- 
tactic (beyond pure semantic) information is 
natural, and has been encouraged by the success in 
theoretical linguistics of grammatical formalisms 
which employ such techniques. Yet the use of 
these techniques in grammar writing, which are 
highly attractive from the point of view of economy 
and expressive power, deprives the generator of 
information that is, strictly speaking, syntactic. 
Semantic heads alone are not sufficient to drive the 
generation process, if syntactic information cannot 
also be made available. Our interim conclusion is 
that strong versions of the lexicalist position do not 
appear to be compatible with our current generator, 
at least for a number of cases. This is not to say 
that it should be abandoned - the benefits in terms 
of clarity and economy are probably too great - but 
some care is needed if it is to be exploited effec- 210 
lively. 
Given that work on this type of generation is in 
its early stages, it is to be hoped that confimfing 
research will enable less restricted grammars to be 
written. Nevertheless, the currently available facili- 
ties have been employed successfully in general, 
mJking it possible to envisage defining the 'ade- 
quacy' of a grammar in terms of its behavior both 
in parsing and in generation. 
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