Building Parallel LTAG for French and Italian 
Made-H616ne Candito 
TALANA & UFRL, Universit6 Paris 7, case 7003, 2, place Jussieu 75251 Paris Cedex 05 France 
marie-helene.candito@linguist.jussieu.fr 
Abstract 
In this paper we view Lexicalized Tree 
Adjoining Grammars as the compilation of a 
more abstract and modular layer of linguistic 
description :the metagrammar (MG). MG 
provides a hierarchical representation of lexico- 
syntactic descriptions and principles that 
capture the well-formedness of lexicalized 
structures, expressed using syntactic functions. 
This makes it possible for a tool to compile an 
instance of MG into an LTAG, automatically 
performing the relevant combinations of 
linguistic phenomena. We then describe the 
instantiation of an MG for Italian and French. 
The work for French was performed starting 
with an existing LTAG, which has been 
augmented as a result. The work for Italian was 
performed by systematic contrast with the 
French MG. The automatic compilation gives 
two parallel LTAG, compatible for multilingual 
NLP applications. 
1. Introduction 
Lexicalized Tree Adjoining Grammars 
(LTAG) is a formalism integrating lexicon and 
grammar (Joshi, 87; Schabes et al, 88) : its 
description units are lexicalized syntactic trees, 
the elementary trees. The formalism is 
associated with a tree-rewriting process that 
links sentences with syntactic structures (in 
either way), by combining the elementary trees 
with two operations, adjunction and substitution. 
We assume the following linguistic features for 
LTAG elementary trees (Kroch & Joshi, 85; 
Abeili6, 91; Frank, 92): 
• lexicalization : elementary trees are anchored 
by at least one lexical item. 
• semantic coherence : the set of lexical items 
on the frontier of an elementary tree forms 
exactly one semantic unit t. 
• large domain of locality : the elementary 
trees anchored by a predicate contain 
positions for the arguments of the predicate. 
This last feature is known as the predicate- 
argument cooccurrence principle (PACP). 
Trees anchored by a predicate represent the 
minimal structure so that positions for all 
arguments are included. These argumental 
positions are extended either by receiving 
substitution or by adjoining at a node. 
Adjunction is used to factor out recursion. 
Figure 1 shows two elementary trees anchored 
by the French verbal form mange (eat-pres-sg), 
whose arguments in the active voice are a 
subject NP and a direct object NP 2. The first 
tree shows all arguments in canonical position. 
The second tree shows a relativized subject and a 
pronominal object (accusative clitic). The 
argumental nodes are numbered, according to 
their oblicity order, by an index starting at 0 in 
the unmarked case (active). So for instance in 
passive trees, the subject is number l, not 0. 
NO* S 
S 
NO V~~O N 1 ,L I 
mange ~~V r t 
qui Cl15 V0 I 
mange 
Figure 1: 2 elementary trees anchored by mange 
Though LTAG units used during derivation are 
lexicalized trees, the LTAG internal 
representation makes use of "pre-lexicalized" 
structures, that we will call tree sketches, whose 
anchor is not instantiated and that are shared by 
several lexicalized trees. The set of tree sketches 
thus forms a syntactic database, in which lexical 
items pick up the structures they can anchor. 
Families group together tree sketches that are 
likely to be selected by the same lexeme: the 
tree sketches may show different surface 
realization of the arguments (pronominal clitic 
realization, extraction of an argument, subject 
inversion...) or different diathesis --matchings 
between semantic arguments and syntactic 
Thus semantically void lexical forms (functional 
words) do not anchor elementary trees on their own. 
And words composing an idiomatic expression are 
multiple anchors of the same elementary tree. 
2 The trees are examples from a French LTAG (Abeill6, 
91), with no VP node (but this is irrelevant here). The 
,1, means the node must receive substitution. The * 
means the node must adjoin in another tree. 
211 
functions-- (active, passive, middle..) or both. 
The lexical forms select their tree sketches by 
indicating one or several families, and features. 
The features may rule out some tree sketches of 
the selected family, either because of 
morphological clash (eg. the passive trees are 
only selected by past participles) or because of 
idiosyncrasies. For instance, the French verb 
peser (to weight) can roughly be encoded as 
selecting the transitive family, but it disallows 
the passive diathesis. 
It remains that tree sketches are large linguistic 
unit. Each represents a combination of linguistic 
descriptions that are encoded separately in other 
formalisms. For instance, a tree sketch is in 
general of depth > 1, and thus corresponds to a 
piece of derivation in a formalism using CF 
rewrite rules (cf (Kasper et al, 95) for the 
presentation of an LTAG as a compiled HPSG). 
This causes redundancy in the set of tree 
sketches, which makes it difficult to write or 
maintain an LTAG. Several authors (Vijay- 
Shanker et al, 92- hereafter (VSS92)- ; Becker, 
93; Evans et al, 95) have proposed practical 
solutions to represent in a compact way an 
LTAG. The idea is to represent canonical trees 
using an inheritance network and to derive 
marked syntactic constructions from base tree 
sketches using lexico-syntactic rules. 
(Candito, 96), building on (VSS92), defines an 
additional layer of linguistic description, called 
the metagrammar (MG), that imposes a general 
organization for syntactic information and 
formalizes the well-formedness of lexicalized 
structures. MG not only provides a general 
overview of the grammar, but also makes it 
possible for a tool to perform automatically the 
combination of smaller linguistic units into a 
tree sketch. 
This process of tree sketch building is 
comparable to a context-free derivation - in the 
generation way- that would build a minimal 
clause. A first difference is that CF derivation is 
performed for each sentence to generate, while 
the tree sketches are built out of an MG at 
compile time. Another difference is that while 
CF derivation uses very local units (CF rules), 
MG uses partial descriptions of trees (Rogers et 
Vijay-Shanker, 94) more suitable for the 
expression of syntactic generalizations. 
MG offers a common, principle-based frame for 
syntactic description, to fill in for different 
languages or domains. In section 2 we present 
the linguistic and formal characteristics of MG 
(in a slightly modified version), in section 3 the 
compilation in an LTAG, and in section 4 we 
describe the instantiation of the MG for French 
and Italian. Finally we give some possible 
applications in section 5. 
2. The metagrammar 
Formally the MG takes up the proposal of 
(VSS92) to represent grammar as a multiple 
inheritance network, whose classes specify 
syntactic structures as partial descriptions of 
trees (Rogers & Vijay-Shanker, 94). While trees 
specify for any pair of nodes either a precedence 
relation or a path of parent relations, these 
partial descriptions of trees, are sets of 
constraints that may leave underspecified the 
relation existing between two nodes. 
The relation between two nodes may be further 
specified, either directly or by inference, by 
adding constraints, either in sub-classes or in 
lateral classes in the inheritance network. 
In the MG, nodes of partial descriptions are 
augmented with feature structures : one for the 
feature structures of the future tree sketches and 
one for the features that are specific to the MG, 
called meta-features. These are, for instance, the 
possible parts of speech of a node or the index 
(cf Section l) in the case of argumental nodes. 
So a class of an instantiated MG may specify the 
following slots : 
• the (ordered) list of direct parent classes 
• a partial description of trees 
• feature structures associated with nodes 3 
Contrary to (VSS92) nodes are global variables 
within the whole inheritance network, and 
classes can add features to nodes without 
involving them in the partial description. 
Inheritance of partial descriptions is monotonic. 
The aim is to be able to build pre-lexicalized 
structures respecting the PACP, and to group 
together structures likely to pertain for the same 
lexeme. In order to achieve this, MG makes use 
of syntactic functions to express either 
monolingual or cross-linguistic generalizations 
(cf the work in LFG, Meaning-Text Theory or 
3 Actually the tree description language --that we will 
not detail here-- involves constants, that name nodes of 
satisfying trees. Several constants may be equal and 
thus name the same node. The equality is either infered 
or explicitly stated in the description. 
212 
Relational Grammar (RG) - see (Blake, 90) for 
an overview). Positing syntactic functions, 
characterized by syntactic properties, allows to 
set parallels between constructions for different 
languages, that are different in surface (for word 
order or morpho-syntactic marking), but that 
share a representation in terms of functional 
dependencies. Within a language, it allows to 
abstract from the different surface realizations 
of a given function and from the different 
diathesis a predicate can show. 
So in MG, subcategorization (hereafter subcat) 
of predicates is expressed as a list of syntactic 
functions, and their possible categories. 
Following RG, an initial subcat is distinguished, 
namely the one for the unmarked case, and is 
modifiable by redistribution of the functions 
associated with the arguments of the predicate. 
Technically, this means that argumental nodes 
in partial descriptions bear a meta-feature 
"initial-function" and a meta-feature "function". 
The "function" value is by default the "initial- 
function" value, but can be revised by 
redistribution. Redistributions, in a broad sense, 
comprise : 
• pure redistributions that do not modify the 
number of arguments (eg. full passive). 
• reductions of the number of arguments (eg. 
agentless passive) 
• augmentations of the number of arguments 
(mainly causative). 
In MG, structures sharing the same initial subcat 
can be grouped to form a set of structures likely 
to be selected by the same lexeme. For verbal 
predicates, a minimal clause is partly represented 
with an ordered list of successive subcats, from 
the initial one to the final one. Minimal clauses 
sharing a final subcat, may differ in the surface 
realizations of the functions. The MG represents 
this repartition of information by imposing a 
three-dimension inheritance network4: 
• dimension 1: initial subcat 
• dimension 2: redistributions of functions 
• dimension 3: surface realizations of 
syntactic functions. 
4 More precisely a hierarchy is defined for each category 
of predicate. Dimension 2 is primarily relevant for 
verbal predicates. Further, remaining structures, for 
instance for argument-less lexemes or for auxiliaries and 
raising verbs are represented in an additional network, 
by classes that may inherit shared properties, but that 
are totally written by hand. 
In an instantiated MG for a given language, each 
terminal class of dimension 1 describes a possible 
initial subcat and describes partially the verbal 
morpho-syntax (the verb may appear with a 
frozen clitic, or a particle in English). Each 
terminal class of dimension 2 describes a list of 
ordered redistributions (including the case of no- 
redistribution). The redistributions may impose a 
verbal morphology (eg. the auxiliary for 
passive). Each terminal class of dimension 3 
represent the surface realization of a function 
(independently of the initial function). For some 
inter-dependent realizations, a class may 
represent the realizations of several functions 
(for instance for clitics in romance languages). 
Terminal classes of the hand-written hierarchy 
are pieces of information that can be combined 
to form a tree sketch that respects the PACP. 
For a given language, some of the terminal 
classes are incompatible. This is stated either by 
the content of the classes themselves or within 
an additional set of language-dependent 
constraints (compatibility constraints). For 
instance a constraint is set for French, to block 
cooccurrence of an inverted subject with an 
object in canonical position (while this is 
possible for Italian). 
3. Compilation of MG to LTAG 
The compilation is a two-step process, 
illustrated figure 2. First the compiler 
automatically creates additional classes of the 
inheritance network : the "crossing classes". 
Then each crossing class is translated into one 
or several tree sketches. 
Hand-Written Hierarchy 
| initial subcat II llsurface realizations oi\[ II funct,o  I 
J U/' I 
' .d~. "s I ) 
(,,,~_~_~_~___ ~ ~IL" " / 
\ "~'~:.',".":'.7~ J ~-"~bd'v'lL, "^uto,,~t~'c ~rat,on v ...... v,~ ~ 
. .'IkL', ~" of cla.,q.~es 
language dependent 
Compatibility 
constraints 
~C~ 
dimension 2 redistributions of 
functions ,¢ 
... "~,,~ Crossing 
Translation into 
LTAG families 
213 
Figure 2 : Compilation of MG to LTAG 
3.1 Automatic extension of the hierarchy 
A crossing class is a linguistic description that 
must fulfill the PACP. Using syntactic functions 
and the three-dimension partition, MG makes 
more precise this well-formedness principle. A 
crossing class is a class of the inheritance 
network that is automatically built as follows: 
• a crossing class inherits exactly one terminal 
class of dimension 1 
• then, a crossing class inherits exactly one 
terminal class of dimension 2 
These two super-classes define an ordered list of 
subcat, from the initial one to the final one. 
• then, a crossing class inherits classes of 
dimension 3, representing the realizations of 
every function of the final subcat. 
Further, for a crossing class to be well-formed, 
all unifications involved during the inheritance 
process must succeed, either for feature 
structures or for partial descriptions. Clashes 
between features or inconsistencies in partial 
descriptions are used to rule out some irrelevant 
crossings of linguistic phenomena. Finally, the 
compatibility constraints must be respected (cf 
Section 2). 
3.2 Translation into LTAG families 
While crossing classes specify a partial 
description with feature structures, LTAG use 
trees. So the compiler takes the "representative" 
tree(s) of the partial description (see Rogers & 
Vijay-Shanker, 94 for a formal definition). 
Intuitively these representative trees are trees 
minimally satisfying the description. There can 
be several for one description. For example, the 
relative order of several nodes may be 
underspecified in a description, and the 
representative trees show every possible order. 
A family is generated by grouping all the trees 
computed from crossing classes that share the 
same class of dimension 1. 
4. Metagrammars for French and 
Italian : a contrast 
We have instantiated the metagrammar for 
French, starting with an existing LTAG (Abeill6, 
91). The recompilation MG---~LTAG insures 
coherence (a phenomena is consistently handled 
through the whole grammar) and completeness 
(all valid crossings are performed). The coverage 
of the grammar has been extended 5. 
Then we have adapted the French MG to Italian, 
to obtain a "parallel" LTAG for Italian, close 
with respect to linguistic analyses. The general 
organization of the MG gives a methodology for 
systematic syntactic contrast. We describe some 
pieces of the inheritance network for French 
and Italian, with particular emphasis on 
dimension 2 and, in dimension 3, on the surface 
realizations of the subject. 
4.1 Dimension 1 
We do not give a description of the content 
of this dimension, but rather focus on the 
differences between the two languages. A first 
difference in dimension 1 is that for Italian, 
there exist verbs without argument 6 
(atmospheric verbs), while for French, a subject 
is obligatory, though maybe impersonal. 
Another difference, is known as the 
unaccusative hypothesis (see (Renzi, 88, vol I) 
for an account). It follows from syntactic 
evidence, that the unique argument of avere- 
selecting intransitives (eg. (I)) and essere- 
selecting intransitives (the unaccusatives, eg. 
(2)) has different behavior when post-verbal: 
(1) *Ne hanno telefonato tre. 
(of-them have phoned three) 
Three of them have phoned 
(2) Ne sono rimaste tre. 
(of-them are remained three) 
Three of them have remained. 
We represent unaccusatives as selecting an 
initial object and no initial subject. A 
redistribution in dimension 2 promotes this 
initial object into a special subject (showing 
subject properties and some object proTperties, 
like the he-licensing shown in (2)). This 
redistribution is also used for specifying passive 
and middle, which both trigger unaccusative 
behavior (see next section). 
s The number of tree sketches passed from 800 to 1100 
lwithout causative trees). 
An alternative analysis would be to consider that these 
verbs select a subject pronoun, that is not realized in 
Italian (pro-drop language). 
7 We take a simpler approach than RG, which accounts 
for most of the Italian data. Unhandled are the auxiliary 
change for verbs, when goal-phrases are added (see 
(Dini, 95) for an analysis in HPSG). 
214 
4.2 Dimension 2 
The MG for French and Italian cover the 
following types of redistribution s : passive, 
middle, causative and impersonal (only for 
French). Causative verbs plus infinitives are 
analysed in Romance as complex predicates. Due 
to a lack of space will not describe their 
encoding in MG here. Figure 3 shows the 
inheritance links of dimension 2 for French 
(without causative). Terminal classes are shown 
without frame. 
V 
~~,7~s,v~ 
Figure 3 : Dimension 2 for French (without causative) 
The verbal morphology is affected by 
redistributions, so it appears in the hierarchy. 
The hierarchy comprises the case of no- 
redistribution, that inherits an active 
morphology : it simply states that the anchor of 
the future tree sketch is also the verb that 
receives inflexions for tense, agreement... 
Refering to the notion of hierarchy of syntactic 
functions (A la Keenan-Comrie), we can say that 
the redistributions shown comprise a subject 
demotion (which can be a deletion) and a 
promotion of an element to subject. 
For active impersonal (3), the subject is demoted 
to object (class SUBJECT---~OBJECT), and the 
impersonal il is introduced as subject (class 
IMPERS---~SUBJECT). 
(3) I1 est arriv6 trois lettres pour vous. 
(IL is arrived three letters for you) 
There arrived three letters for you. 
Passive is characterized by a particular 
morphology (auxiliary bearing inflections + past 
participle) and the demotion of subject (which is 
either deleted, class SUBJECT--->EMPTY, or 
demoted to a by-phrase, class SUBJECT--~AGT- 
OBJ), but not necessarily by a promotion of the 
object to subject (class OBJECT---->SUBJECT) (cf 
(Comrie, 77)). In French, the alternative to 
object promotion is the introduction of the 
impersonal subject (class IMPERS---~SUBJECT )9. 
This gives four possibilities, agentless personal 
(4), full personal (5), agentless impersonal (6), 
full impersonal, but this last possibility is not 
well attested. 
(4) Le film sera projet6 mardi prochain. 
The movie will be shown next tuesday. 
(5) La voiture a 6t6 doubl6e par un v61o. 
The car was overtaken by a bike. 
(6) I1 a 6t6 d6cr6t6 l'6tat d'urgence. 
(IL was declared the state of emergency) 
The state of emergency was declared. 
Middle is characterized by a deletion of the 
subject, and a middle morphology (a reflexive 
clitic se). Here also we have the alternative 
OBJECT--~SUBJECT (7) or IMPERS--->SUBJECT 
(8). The interpretation is generic or deontic in French. 
(7) Le th6 se sert ~ 5h. 
(Tea SE serves at 5.) 
One should serve tea at 5. 
(8) I1 se dit des horreurs ici. 
(IL SE says horrible things here) 
Horrible things are pronounced in here. 
Now let us contrast this hierarchy with the one 
for Italian. Figure 4 shows dimension 2 for 
Italian. 
l 
~OBJECT ~ EX'ITc.~DED- S O BJ ECT i 
PERSONAL PASSIVE 
Figure 4 : Dimension 2 for Italian (without causative) 
In Italian, what is called impersonal (9a) is a 
special realization of subject (by a clitic sO, 
meaning either people, one or we. (cf 
Monachesi, 95). The French equivalent is the 
8 The locative alternation (John loaded the truck with 
oranges/John loaded oranges into the truck), is not 
covered at present time, but can easily be added. It 
requires to choose an initial subcat for the verb. 
9 So we do not analyse impersonal passive as passive to 
which apply impersonal. This allows to account for the 
(rare) cases of impersonal passives with no personal 
passive counterpart. 
215 
nominative clitic on (9b). 
(9a) it. Si parti. 
(SI left) People / we left. 
(9b) fr. On partit. 
This impersonal si is thus coded as a realization 
of subject, in dimension 3, and we have no 
IMPERS---~SUBJECT promotion for the Italian 
dimension 2. The impersonal si can appear with 
all redistributions except the middle. The Italian 
middle is similar to French, with a reflexive 
clitic si. Indeed impersonal si, with transitive 
verbs and singular object (10), is ambiguous with 
a middle analysis (and subject inversion). 
(10) Si mangia il gelato. 
(SI eat-3sg the ice-cream) 
The ice-cream is eaten. 
With a plural nominal object, some speakers do 
not accept impersonal (with singular verb (11 a)) 
but only the middle (with verb agreement (1 lb)). 
(1 la) Si mangia le mele. 
(SI eat-3sg the apples) 
(1 lb) Si mangiano le mele. 
(SI eat-3pl the apples) 
Another difference with French redistributions, 
is that when the object is promoted, in passive 
or middle, it is as a subject showing unaccusative 
behavior (eg. he-licensing, cf section 4.1). To 
represent this, we use the class 
OBJECT---~EXTENDED-SUBJECT, which is also 
used for the spontaneous promotion of initial 
object of unaccusative intransitives (cf section 
4.1). So for Italian, passive (agentless or full) 
and middle (1 lb) comprise a subject demotion (a 
mandatory deletion for middle) and the 
promotion OBJECT--~EXTENDED-SUBJECT, 
while for intransitive unaccusatives, this 
promotion is spontaneous. 
Other differences between French and Italian 
concern the interaction of causative with other 
redistributions : passive and middle can apply 
after causative in Italian, but not in French. 
4.3 Dimension 3 
We describe in dimension 3 the classes for the 
surface realizations of subject. This function is 
special as it partially imposes the mode of the 
clause. The subject is empty for infinitives and 
imperatives I°. Adnominal participial clauses are 
to See (Abeill~, 91) for the detail of the linguistic 
analyses chosen for French. We describe here the 
hierarchical organization. 
represented as auxiliary trees that adjoin on a N, 
the subject is the foot node of the auxiliary tree 
(we do not detail here the different participial 
clauses). 
For French (Figure 5), when realized, the subject 
is either sentential, nominal or pronominal 
(clitic). Nominal subjects may be in preverbal 
position or inverted, relativized or cleft. These 
last two realizations inherit also classes 
describing relative clauses and cleft clauses. 
Sentential subjects are here only preverbal. Clitic 
subjects are preverbal (post-verbal subject clitics 
are not shown here, as their analysis is special). 
Note that in dimension 2, the class 
IMPERS---~SUBJECT specifies that the subject is 
clitic, and dominates the word il. This will only 
be compatible with the clitic subject realization. 
/ I~ON.R~,~Z~,E~ t~,U~'~j~ ~ 
k SUBJECT 
Figure 5 : SubJect realizations for French 
For Italian, (Figure 6), the hierarchy for subjects 
is almost the same : a class for non-realized 
subjects is added, since Italian is a pro-drop 
language, and pronominal subjects are not 
realized. But we mentioned in section 4.2 the 
special case of the impersonal subject clitic si. 
To handle this clitic, the Italian class for clitic 
subject introduces the si. 
Figure 6 : Subject realizations for Italian 
(differences with French in bold) 
216 
5. Applications 
The two LTAG for French and Italian are easy 
to maintain, due to the hierarchical 
representation in MG. They can be customized 
for language domains, by cutting subgraphs of 
the inheritance network in MG. 
The MG for French is currently used to maintain 
the French LTAG. It has also been used to 
generate the tree sketches for the text generator 
G-TAG (Danlos & Meunier, 96), based on TAG. 
The generator makes use of tree sketches 
characterization as a set of features ---called t- 
features- such as <passive>, <infinitival- 
clause>... This characterization has been 
straightforward to obtain with the 
representation of the tree sketches in MG. 
Further, the two MG for French and Italian can 
provide a basis for tranfer between syntactic 
structures for Machine Translation. LTAG 
elementary trees correspond to a semantic unit, 
with (extendable) positions for the semantic 
arguments if any. (Abeill6, et al, 90) propose to 
pair elementary trees for the source and target 
languages and to match in these pairs the 
argumental positions of the predicate. Once 
these links are established, the synchronous 
TAG procedure can be used for translation. 
The argumental positions correspondance is 
straightforward to state within the MG 
framework. We plan to define an automatic 
procedure of tree-to-tree matching using MG 
representations for source and target languages, 
once the initial functions of arguments are 
matched for pairs of predicates. This procedure 
will make use of sets of t-features to 
characterize tree sketches (as in G-TAG) derived 
at the MG--->LTAG compilation time. 
Correspondances between t-features or sets of 
t-features have to be defined. 

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