Generating French with a Reversible Unification Grammar 
Dominique Estival 
ISSCO 
54 rte des Acacias, CH-1227 Geneva 
O. Intr~cluction 
In this paper, we describe the linguistic solutions 
to some of the problems encountered in writing a 
reversible French grammar. This grammar is pri- 
marily intended to be one of the components of a 
machine translation system built using ELU, 1 an 
enhanced PATR-II style unification grammar linguis- 
tic environment based on the LID system described in 
Johnson and Rosner (1989), but it is also part of our 
more general experimentation with fully reversible 
grammars. 
The requirement that it be reversible imposes a 
stringent criterion of linguistic adequacy on a gram- 
mar, siuce it is not allowed to overgenerate while it 
must at the same time provide a large coverage for 
analysis (Dymetman and IsabeUe (1988)). Formally, 
grammars that are fully reversible must be completely 
declarative, since no reierence can be made in the 
grammar rules to the process (analyzer or synthesizer) 
which will use them. The unification formalism 
makes itt possible to write such grammar statements, 
because due to the associativity and commutativity of 
the unitication operation, the result of unifying feature 
structures is independent of the order in which they 
are unitied (Appelt (1989)). 
Writing reversible grammars, however, presents 
problems which do not arise in the traditional gram- 
mars used for either analysis or generation. In addi- 
tion, the progress accomplished recently in building 
generators for unification grammars has already 
revealed some of the problems posed by unification- 
based reversible grammars. 2 As shown by Russell et 
al. (1990), even though the grammar rules do not refer 
to the generation process, the generation algorithm 
imposes particular constraints on the grammar formal- 
ism. 3 
This paper concentrates particularly on the prob- 
lems encountered in the generation of French, 
specifically in the analysis to be given to clitics. 
1 ELU stands for Environnement Linguistique 
d' Unification. 
2 See Dymetman and Isabelle (1988), Shieber (1988), 
Shieber et al. (1989), van Noord (1988), inter alia. 
3 For instance, we cannot compare our grammar with the 
one pre~;nted in Saint-Dizier (1989), since his grammar is nei- 
ther reversible nor purely declarative, as the rules are annotat- 
ed with ' generation points'. 
We first briefly describe the aspects of the genera- 
tion algorithm and of the grammar formalism which 
are relevant to the particular problems under discus- 
sion, then present the facts of French syntax which 
pose those problems and the solutions we have 
adopted. 
1. The Generator 
The generation algorithm of ELU is based on the 
algorithm described in Shieber et al. (1989) and was 
developed at ISSCO by J. Carroll. 4 Generation is 
head-driven: each role has a "semantic head" (see 
Shieber (1988)), which is specified by the ~ammar 
writer, and the head daughter of a rule is generated 
before its siblings. The depth-first algorithm defines a 
downward path through the semantic heads of rules. 
This algorithm does not require that the grammar 
be semantically monotonic. Non-monoto\]tficity is 
obtained by having the generator distinguish two types 
of rules in the grammar, "chaining" arKl "non- 
chaining" rules, and by introducing the notion of 
"pivot". Following from this distinction, it employs 
both bottom-up and top-down processing. 
The partition of the set of grammar rules into 
chaining and non-chaining rules is pre-compiled from 
the specification of what counts as the "semantics" of 
a feature structure. In a chaining rule, the mother and 
the head daughter have identical semantics; chaining 
rules are used bottom-up from the "pivot", which is 
defined as the lowest point in the path through the 
head daughters of chaining rules at which the seman- 
tics of the feature structure remains unchanged. In a 
non-chaining rule, the mother and the head daughter 
have different semantics; non-chaining rules are used 
top-down from the pivot. 
The efficiency of the ELU generator depends in a 
large part on the restrictors defined by the grammar 
writer. Computing the pivot, i.e. creating a teachabil- 
ity table for chaining rules, and bottom-up processing 
4 Cf. Russell et al. (1990) for a description of the differ- 
ences between the two algorithms. 
106 1 
are both controlled by pre-compiled "linking" inforo 
mation, which is encoded as sets of restrictor values. 
A restrictor is a specification of a value wlfich can be 
computed from a feature structure (syntactic category, 
for example, is often defined as a resUictor). Before 
attempting unification between two feature structures, 
the values tbr the restrictors are checked in both of 
them; if these values are not compatible, unitication 
would be bound to fail mid is not tried. As linking 
information is only relevant for chaining rules, it is 
only used bottom-up during processing, ~md since by 
definition, chaining rules have the same semantics for 
their heads, linking information must be syntactic. 
Restrictors are also used heavily m the selection of 
lexical items, so the attributes chosen as restrictors 
have to be good discriminauts between i~ature struc- 
tures. 5 
"Ihe generation algorithm by itself guarantees nei- 
ther the completeness nor the coherence of tile result- 
ing feature structure. The responsibility tbr prevent° 
ing the generation of structures which unify with the 
input, bu~ me incomplete (i.e. ensuring completeness) 
rests with the grmnmar writer: any structure which 
needs to be generated in its entirety should not be 
represented as an uncotx~trained ti~ature structure, but 
must be specified as another data type, i,e. a list, a 
tree, or a user-defined type expression. Tt~e graulmar 
writer and the generator share tile responsibility for 
preventing additions to the input structure (i.e. 
preserving coherence): the gramm~ writer must 
again select the appropriate data types, and the gen- 
erator "tTcezes" uninstantiated variables that occur in 
the input. 
The choice of appropriate data types as well as of 
good restrictors is therclore crucial to ensure flint the 
grammar is not only efficient but usable in generation. 
2. The G, rammar Formalism. 
The syntactic ieplesentations built by file parser 
are trees where each node is a directed acyclic graph 
consisting of atuibute-value pairs (i.e. a feature struc~ 
ture which allows reentrancy). 'File semantic 
represemations used ~s input by the generator are 
feature structures derived from the syntactic trees. 
The gr~unmar rules consist of context-free phrase 
structure rules annotated with corrstraint equatiotrs 
expressing relations between the categories mentioned 
ill the rule. The ELU tormalism provides a generali- 
zation of the template facility of PATR-II, the "rela- 
tional abstractions", which are statements abstracting 
over sets of constraint equations. These statements 
5 Restrictors are also used to restrict the search space in 
parsing (see Shieber (1985)). "fbe use of linking information 
in generation was first proposed by van Noord (1988). 
may receive multiple and mcursive definitions. To 
give multiple definitions to a relational abstraction 
permits collapsing what i~l an unextended PATR-Iike 
formalism would be several distinct rules, and is a 
powerful way to capture linguistic generalizations. 
Multiple definitions, however, give rise to a high 
degree of non-determinism during processing. There- 
fbre, while the parser expands multiple definitions 
whenever they are encountered, the generator uses a 
lazier approach and only expands them when they are 
needed. Nevertheless, tiffs strategy is not sufficient, 
and the problem posed by the non-determinism of 
relational abstractions is the most complex and severe 
of the grammar/generator interactions described in 
Russell et al. (1990), because of its adverse effects on 
the restliclion of top-down generation. 
3. French Critics 
Any French gr~wnmar must account for the posi- 
tion aid ordering of preverbal clitics. While full com- 
plement aud modifier ptwases occur to tile right of the 
mah~ verb of a clause, up to three elitics may occur in 
front of a verb, as in (1). 
(1) 11 m'y en a fait p,'u-t. 
he me there of it informed 
He infot~ned me of it there. 
Moreover, tile clitics must appear in a fixed order, 
which, as shown ill (2), is independent of the seman- 
tics of the critics. 
(2) a. Ils vous l'y ont dolm~e. 
they to you it there gave 
They gave it to yott there. 
b. *Ils leur l'y ont dotm6e. 
they to them it there gave 
77wy gm, e it to them there. 
This f~ed order can be represented by the traditional 
table given in (3). 6 
(3) Ordering of French ditics 
me le lui y en 
te la leur 
se les 
nous 
vous 
in most accounts of the distribution shown in (3), tile 
problem is simplified, because only subcategolized 
complements are de'tit with. A French preverbal eli- 
tic, however, is not necessarily a subcategorized com- 
plement of the verb; adverbials and parts of comple- 
ment phrases can also cliticize, and the grammatical 
category of some clitics is that of adverbs or 
6 In (3), se stands for any of the so-called 'R-clitics', i.e. 
the reflexive and reciprocal pronouns, as well as the inherent 
reflexive ~md the middle marker, as explained in more detail 
below. 
2 107 
quantifiers. 
The contrast between (4.a) and (4.b) shows that a 
clitic can be either a full complement, or part of a 
complement. In (4.a), en is the full prepositional 
object of the verb parler, while in (4.b), en represents 
the partitive prepositional phrase which is the comple- 
ment of the object of vouloir. 
(4) a. I1 en parlait souvent. 
he often talked about it 
\[cf. I1 parlait souvent de ce Uvre\] 
\[he often talked about that book\] 
b. J' en veux deux. 
I want two of them 
\[cf. Je veux deux de ces pommes\] 
\[I want two of these apples\] 
The contrast between (5.a) and (5.b) shows that a eli- 
tic can either be subcategorized or not. In (5.a), y is 
the subcategorized complement of the verb aller, 
while in (5.b), y is a locative adverb, which is not sub- 
categorized by the verb dormir. 
(5) a. I1 y aUait souvent. 
he often went there 
\[cf. I1 allait souvent dans cette ville\] 
\[he often went to that city\] 
b. I1 y dormait souvent. 
he often slept there. 
\[cf. I1 dormait souvent dans cet hdtel\] 
\[he often slept in that hotel\] 
Besides the personal pronouns and the adverbs 
given in Table (3), there are other lexical items which 
are not usually considered in the treatment of French 
clitics, but whose behavior is closely related. 7 The 
negative elements pas, plus, jamais, rien, and the 
quantifiers tant, autant, plus, moins and tout 8 also cli- 
ticize and may appear preverbally. 
While ,all the clifics of Table (3) must appear in 
front of the traditional AUX constituent (i.e. before 
any of the verbal elements of the VP), the examples in 
(6) slhow that the elements of this second set appear 
inside AUX, more precisely after the first tense- 
bearing verbal form. 
(6) a. I1 n'en avaitjamais 4t6 persuad6, 
he had never been sure of it 
b. I1 n' en avait jamais rien cru. 
he had never believed any of it 
c. Je n'y en ai jamais autant vu. 
I had never seen so many of them there 
7 They are, however, the subject of work in theoretical 
linguistics, see e.g. Perlmutter (197l), Emonds (1975), Kayne 
(1975), and more recently Pollock (1989). Interestingly, 
though it was developed in a different framework and for dif- 
ferent reasons, our treatment of those elements is compatible 
with the latter's analysis (cf. also fn.9). 
8 The quantifier tout has actually several forms, inflected 
for gender and number: tout/tousltoute/toutes. 
There are thus at least two slots for clitics inside a 
French VP, and neither of these slots correlates with 
argumenthood. The quantifiers rien and autant which 
appear inside AUX in (6.b) and (6.c) are (paJtts) of the 
argument of the verbs croire and voir, and so is the 
quantifier en, which is in front of the AUX. On the 
other hand, the adverb y is not an argument in (6.c), 
nor is jamais in (6.a-c). 
Therefore, the lexical entry of every clitic element 
must specify not only that it is a clitic but whether it 
appears in front of or inside the AUX constituent. 
4. Generation 
Theoretically, the fundamental problem posed by 
clitics stems from their dual nature, syntactic' and mor- 
phological, and partly consists in deciding whether to 
treat them by syntactic or by morphological 
processes. 9 
Descriptively, there are three issues to be 
addressed: argument-binding, linear ordering rela- 
tior~s, and categorial status of the clitics. All three 
give rise to problems in generation due to non- 
determinism, for which the solution is to ensure that 
the lexical verb is instantiated as soon as possible. 
4.1. Subcategorizatlon 
The unification formalism makes it very natural to 
encode syntactic information in the lexicon and with a 
lexicalist approach, our treatment of arguments is 
straightforward: we make the standard use of a sub- 
categorization list to encode the complements a verb 
requires. Since any complement phrase may be real- 
ized as a clitic, this fact is not mentioned in the sub- 
categorization list. to 
9 E.g., restrictions on coordination show that clitics are not 
independent syntactic constituents. 
(i) * I1 me et te connait. 
he knows me and you 
Cf. the various analyses presented in Borer (1986). More re- 
cently (Rizzi and Roberts (1989), Kayne (1990)), the question 
has been reformulated in terms of the type of mechanism (ad- 
junction or substitution) involved in cliticization and of 
whether clitics are phrasal heads or not. With the lexicalist ap- 
proach adopted in our grammars both types of processes can 
be referred to in the lexicon, but it is of course still desirable 
that the two be clearly separated. 
l0 This analysis contrast with that of Baschung et al. 
(1987), or B6s et al. (1989), which treats separately comple- 
ments appearing to the left and complements appearing to the 
right of the verb. Their reason for doing so is that they take 
the variants shown in (i) and (ii) to indicate a relatively free 
order of complements (sulx-ategorized or not) in French. 
(i) il a donn6 (hier) un livre ~t Marie (hier). 
yesterday he gave a book to Marie 
(ii) il a donn6 (hier) ?l Marie un livre (bier). 
yesterday he gave a book to Marie 
While the ordering of full complements inside the VP poses 
some problems for generation, it is a separate question from 
that of cliticization, and the two should receive principled 
solutions of their own. 
108 3 
During analysis, art element found in the VP is 
checked against Subcat, the subcategorization list of 
the predicate. If it does not unify with any element of 
Subcat, ~t is treated as a VP modifier and added to 
Mods, the list of modifiers. From the point of view of 
generation, clitics realize elements from either the 
Subcat list or from the Mods list. 
For instance, we partly follow the lexicalist 
analysis of Grimshaw (1982) for the R-clitics 
represented by se. That is, we consider that the R- 
clitic is not an argument of "iuheienfly reflexive" 
(7.a,b) and "middle" verbs (7.c), but a morpho- 
syntaclic marker, t i 
(7) a. i~1 s'est 6vanoui. 
he fainted 
b. I1 se le demandait. 
he was wondering about it 
c. II s'est cassd. 
it broke 
But in reciprocal and true reflexive constructions, such 
as (8.a,b), we treat the R-clitic as a pronoun which is 
an argument of the verb. 12 
(8) a. Ils se sont regardds. 
they looked at each other~themselves 
b. Ils se les sont donnds. 
they gave them to each other~to themselves 
Thereibre, because the verbs in the examples of (7) 
are marked in the lexicon as being iitherently 
reflexive, an R-clitic is generated from Subcat without 
being bound to the list of semantic arguments. In (8) 
on the other hand, the verbs are respectively transitive 
and ditransitive: in their case, a semantic argument is 
both bound to ,an element of Subcat and re,alized as a 
reflexive pronoun because of its own semantic 
features. In (9.a-c) se is, as in (7), the inherent 
reflexive marker mid is generated from Subcat. In 
(9.a) en is the partitive phrase of a subcategorized 
argument; y in (9.b) is a subcategorized locative argu- 
ment from Subcat and in (9.c), it is a VP adverb from 
Mods. 
(9) a. I1 s'en est cass6 deux. 
two of them broke 
b. Ils s'y trouvaient. 
they were there 
c. lls s'y vendaient. 
they were soM there 
As described in Russell et at. (1990), problems 
arise iin generation because of non-determinism and 
becau~e of the unavailability of some syntactic infor- 
mation to the generator. The subcategorization list 
II In (7.a), there is no non-reflexive verb e~'anouir, and in 
(7.b), die reflexive verb has a different semantics than the 
non-reflexive verb from which it is lexically derived. 
12 In this respect, our analysis also differs from that 
presented in Wehrli (1986). 
mechanism typical of unification grammars is a source 
of both these kinds of problems. Subcategorization 
lists are relational abstractions with multiple 
definitions; therefore, they introduce non-detenninism 
in tile expansion of the rules in which they are 
invoked. Moreover, they exemplify the type of syno 
tactic information typically found ill lexical entries; 
tiffs infommtion is not available to the generator until 
the lexical head has been instantiated, but if it was 
avMlable at a higher point in the path through the rules 
it would help constrain the top-down search. 
In particular, here, separating the elements found 
inside the VP into arguments and modifiers can only 
be (lone alter the lexical head has been instantiated 
mid its subcategorization list is available. As shown 
by the two meanings of the verb trouver given in the 
lexical entries (10.a,c) and exemplified in (10.b,d), the 
semantics of the verb (its argument list) may change 
according to its subcategorization list. 
(10)a. trouver * v {+UN} 
!Verb Imain !avoir ~Nrefl !trans 
!Subcat(np,np) 
b. I1 l' y trouve. 
he finds it there 
\[cf. II le trouve dans les Alpes.\] 
\[he finds it in the Alps\] 
c. trouver * v {+UN) 
!Verb !main !~tre IRefl !intrans 
!Subcat(np,pp) !PPsem(loc) 
d. I1 s'y trouve. 
it is located there 
\[cf. I1 se trouve dans les Alpes.\] 
\[it is located in the Alps\] 
In (10.d) the clitic y is ,an argument (i.e. it is bound to 
one of the variables in the arguments list), while in 
(10.b) it is not (i.e. it is added to the modifiers list). 
Even though the two possiblities ,are mutually 
exclusive, if the subcat list is not available at the VP 
level, the search must proceed top-down and tim VP is 
expanded top-down and non-deterministically. Recall 
that when the semantics for the head daughter of a 
rule does not change, the rule is a chaining rule which 
is used bottom-up, but if the semantics of the head 
changes, then the rule is a non-chaining rule, which is 
used top-down and defines a pivot. 
4.2. Linear ordering 
As was shown by the examples of (2), the linear 
ordering among preverbal clitics is independent of 
their semantics; it is also independent of the syntactic 
features of their dominating clause, i.e. negation, 
inversion, etc, A perspicuous way to express citric 
ordering is to have one relational abstraction with 
separate definitions stating the different precedence 
constraints holding between two preverbal clitics. 
The simplified definitions for Precede(C1,C2) given 
in (11) would account for most of the distribution 
4 109 
facts of Table (3) in a natural and elegant way. 13 
(11) Precede(C 1 ,C2) 
<C1 head morph pers> = 1/2 
<C2 head morph pers> = 3 
Precede(C1,C2) 
<C1 head morph case> = acc/refl 
<(.'2 head morph case> = dat 
Precede(C1,C2) 
<C1 head morph case> = refl 
<C2 head morph case> = acc\]dat 
Precede(C1,C2) 
<C1 head sem pred> = y 
<(~ head sem pred> = en 
4.3. Categorial status 
A characteristic property of clitics is that they do 
not have a maximal projection and remain X ° consti- 
tuents, with their own syntactic category feature com- 
ing from the lexicon. To express the fact that a dative 
pronoun or the clitics y and en actually stand for a PP 
can be done by building a PP in the lexicon, e.g. with 
a relational abstraction such as Make-PP(CI,PP). 
(12) Make-PP(CI,PP) 
<CI head sem pred> = "y/en 
<CI head morph case> = dat 
<PP head sere pred> = ~t 
<PP head sem args> = \[<CI>\] 
Make-PP(CI,PP) 
<CI head sem pred> = y 
<PP head sere pred> = 
<PP head sem args> = \[<CI>\] 
Make-PP(CI,PP) 
<C1 head sem pred> = en 
<PP head sere pred> = de 
<PP head sere args> = \[<CI>\] 
The relational abstractions Precede and Make-PP 
constitute an elegant collapsing of syntactic and lexi- 
cal rules which is useful in analysis: the grammar 
rules which rewrite VPs containing clitics need not 
specify all the various possibilities. However, as with 
the relmional abstractions encoding subcategorization 
facts, its multiple definitions render Precede non- 
deterministic. The non-determinism of Make-PP, 
which is due to the fact that some clitic forms are 
ambiguous, is no less severe. During generation, the 
evaluation of the equations is delayed until the seman- 
tics for the head has been instantiated, and if the lexi- 
cal head is not instantiated early enough, rules which 
involve these relational abstractions are tried repeat- 
13 There are other constraints not accounted for by (11), 
e.g. the one requiring that an ambiguous acc/dat form cannot 
be interpreted as an accusative in front of a dative: 
(i) ,k Elie nous lui prdsentera. 
she us to him will introduce 
Similar constraints exist among the clitic elements appearing 
in post-verbal position. 
edly even if they cannot apply. 
In conclusion, for the purpose of generation, we 
need an analysis where the semantic head of the VP is 
not necessarily the lexical main verb, but is the ele- 
ment which will be sure to be instantiated as early as 
possible. In an analysis reminiscent of current work 
in the Government-Binding framework, 14 where a 
clause is IP (Inflectional Phrase), the maximal projec- 
tion of INFL, we take as the semantic head for our 
rules the element which bears tense. This element, I, 
may be either the main verb or an auxiliary which 
takes the main verb as complement. 
With this analysis of the structure of VP, the 
semantics of the head daughter I remains the same 
along the path through the semantic heads so that the 
pivot of the structure, i.e. the point at which bottom- 
up generation can start from, is at the end of path. At 
that point, either I is the main verb (V-raising has 
applied) and it can be instantiated immediately, or I is 
an auxiliary (V-raising hasn't applied) and the main 
verb is its sister, which can be reached through other 
chain rules. 
We can deal with clitics in two ways: 
• by successive adjunction to the head: 
(13) TP 
/ \ 
el TP 
/ \ 
cl TP 
• by building a list, itself adjoined to the head: 
(14) TP 
/ \ 
CL TP 
/ \ 
cl cl 
Besides being descriptively more adequate, since the 
ordering constraints hold between the clitics them- 
selves, not between a clitic and a verbal constituent, 
the second approach is to be preferred because a list 
ensures completeness of the resulting feature struc- 
ture. Moreover, the whole list of clitics can he built 
without instantiating the lexical verbal head. With the 
two clitic positions and taking I as the head, the syn- 
tactic structure for a VP is as in (15). 
(15) T\]? 
/ \ 
CL TP 
/ I \ 
I CL V2 
/ \ 
verb . . . 
14 Cf. fn.6 and 9, and work cited in the references given 
there. 
5 Ii0 
Clitic elements are marked as to whether they 
must appear to the left or to the fight of I. If V-raising 
hasn't applied, as in the examples of (6), the two critic 
lists will be in front of the main verb, on either side of 
I. If V-raising has applied, the two clitic fists will still 
be on either side of I, and of the main verb, as in (16). 
(16) a. I1 ne l'en persuaderajamais. 
he will never convince her of it 
b. I1 n' en croit jamais rien. 
he never believes any of it 
c. Je n ' y en voit jamais autant. 
I never see so many of them there 
5. Conch~slon 
We have shown with die example of French clitics 
how some problems inherent in the writing of reversi- 
ble grammars arise, ~md what aspects of the formalism 
are responsible for them. The solutions we propose 
are motivated by internal considerations ,and provides 
a coherent syntactic account of the phenomena under 
consideration, i.e. clitic placement and so-called 
"adverb climbing" (although space prevents us from 
showing tile details here, riley also deal adequately 
with vefibal negation). These solutions make full use 
of the properties and adwmtages of die lexicalist 
approach to gr,'unmars while circumventing (some of) 
the dange~ it presents. 
** I ,'un grateful to Susan Warwick and Graham 
Russell for the time they have spent helping me under- 
stand EHJ and its generator. Neither of them, of 
course, is responsible for any mistake in this paper. 

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