A principle-based hierarchical representation of 
Marie-H616ne Candito 
TALANA 
Universit6 Paris 7 Tour centrale 86me 6tage pi6ce 801 
75251 Paris cedex 05 FRANCE 
e-mail : marie-helene.candito@linguist.jussieu.fr 
LTAGs 
Abstract 
Lexicalized Tree Adjoining Grammars 
have proved useful for NLP. However, 
numerous redundancy problems face 
LTAGs developers, as highlighted by Vijay- 
Shanker and Schabes (92). 
We present a compact hierarchical 
organization of syntactic descriptions, that 
is linguistically motivated and a tool that 
automatically generates the tree families of 
an LTAG. The tool starts from the syntactic 
hierarchy and principles of well-formedness 
and carries out all the relevant combinations 
of linguistic phenomena. 
1 Lexicalized TAGs 
Lexicalized Tree Adjoining Grammar (LTAG) is 
a formalism integrating lexicon and grammar (Joshi, 
87; Schabes et al., 88). It has both linguistic 
advantages (e.g elegant handling of unbounded 
dependencies and idioms) and computational 
advantages, particularly due to lexicalization 
(Schabes et al., 88). Linguists have developed over 
the years sizeable LTAG grammars, especially for 
English (XTAG group, 95; Abeill6 et al., 90) and 
French (Abeill6, 91). 
In this formalism, the lexical items are 
associated with the syntactic structures in which 
they can appear. The structures are lexicalized 
elementary trees, namely containing at least one 
lexical node at the frontier (called the anchor of the 
tree). The elementary tree describes the maximal 
projection of the anchor. So a verb-anchored tree has 
a sentential root. Features structures are associated 
with the trees, that are combined with substitution 
and adjunction. Adjunction allows the extended 
domain of locality of the formalism : all trees 
anchored by a predicate contains nodes for all its 
arguments. 
Such a lexicalized formalism needs a practical 
organization. LTAGs consist of a morphological 
lexicon, a syntactic qexicon of lemmas and a set of 
tree schemata, i.e. trees in which the lexical anchor is 
missing. In the syntactic lexicon, lemmas select the 
tree schemata they can anchor. When the grammar is 
used for parsing for instance, the words of the 
sentence to be parsed are associated with the 
relevant tree schemata to form complete lexicalized 
trees. 
Tile set of tree schemata forms the syntactic part 
of the grammar. The tree schemata selected by 
predicative items are grouped into families, and 
collectively selected. A tree family contains the 
different possible trees for a given canonical 
subcategorization (or predicate-argument structure). 
The arguments are numbered, starting at 0 for the 
canonical subject. Along with the "canonical" trees, a 
family contains the ones that would be 
transformationally related in a movement-base 
approach. These are first the trees where a 
"redistribution" of the syntactic function of the 
arguments has occurred, for instance the passive 
trees, or middle (for French) or dative shift (for 
English), leading to an "actual subcategorization" 
different from the canonical one. When such a 
redistribution occurs, the syntactic function of the 
arguments change (or the argument may not be 
realized anymore, as in the agentless passive). For 
instance, the subject of a passive tree is number l, 
and not 0 (figure 1). This is useful from a semantic 
point of view, in the case of selectional restrictions 
attached to the lexical items, or of a 
syntactic/semantic interface. 
s s 
N0$ V0 N15 N15 Vm$ V0 
P N0$ I 
par 
Figure 1. Declarative transitive tree and corresponding full 
passive for French 1 
And secondly, a family may contain the trees 
with extracted argument (or cliticized in French). 
There are different types of trees for extraction. \]n 
the English grammar for instance, there are trees for 
wh-questions and trees for relative clauses (that are 
adjoined to NPs). In the French grammar there are 
also separate trees for cleft sentences with gaps in 
the clause, while the corresponding it-clefts are 
handled as relative clauses in the English grammar. 
Nr 
NO* S 
C V0 N15 
I qui 
S 
N0$ Vr 
Cll~V0 
Figure 2. Two trees of the strict transitive family for French : the relativized subject and the cliticized object. 
1The French LTAG comprises trees with flat structure (no VP node); in the passive tree, the auxiliary is substituted; the same 
symbol N is used for nominal phrases and nouns, the difference being expressed with a feature <det> (Abeill6, 91). we do not 
show the feature equations for the sake of clarity. For the French grammar, the average number of equations per tree is 12. 
194 
So a family contains all the schemata for a given 
canonical subcategorization. Yet, in the syntactic 
lexicon, a particular lemma may select a family only 
partially. For instance a lemma might select the 
transitive family, ruling out the passive trees. 
On the other hand, the features appearing in the 
tree schemata are common to every lemma selecting 
these trees. The idiosyncratic features (attached to 
the anchor or upper in the tree) are introduced in the 
syntactic lexicon. 
2 Development and maintenance 
problems with LTAGs 
This extreme lexicalization entails that a 
sizeable I,TAG comprises hundreds of elementary 
trees (over 600 for the cited large grammars). And as 
highlighted by Vijay-Shanker and Schabes (92), 
information on syntactic structures and associated 
features equations is repeated in dozens of tree 
schemata (hundreds for subjecbverb agreement for 
instance). 
This redundancy problem is present at all levels 
of grammar development. The writing of an I,TAG is 
a rather fastidious task; its extension and/or 
maintenance is very difficult, since maintaining the 
grammar means for instance adding an equation to 
hundreds of trees. Extending it means adding new 
tre, es along with their equations, and it can also 
entail the addition of new features in existing trees. 
Furthermore, the amount of work may grow 
exponentially with the size of the grammar, since all 
combinations of phenomena must be handled. 
And finally, in addition to the practical 
problems of grammar writing, updating and storage, 
redundancy makes it hard to get a clear vision of the 
theoretical and practical choices on which the 
grammar is based. 
3 Existing solutions 
A few solutions have been proposed for the 
problems described above. Solutions to the 
redundancy problem make use of two tools for 
lexicon representation : inheritance networks and 
lexical rules. Vijay-Shanker and Schabes (92) have 
first proposed a scheme for the efficient 
representation of LTAGs, more precisely of the tree 
schemata of an I.TAG. They have thought of a 
monotonous inheritance network to represent the 
elementary trees, using partial descriptions of trees 
(Rogers and Vijay-Shanker, 92 and 94) (see section 
4.1 for further detail). They also propose to use 
"lexical and syntactic rules" to derive new entries. 
The core hierarchy should represent the "canonical 
trees", and the rules derive the ones with 
redistribution of the functions of arguments (passive, 
dative shift...) and the ones with extracted argument 
Becker (93; 95) also proposes a hybrid system 
with the same dichotomy : inheritance network for 
the dimension of canonical subcategorization frame 
and "meta-rules" for redistribution or extraction (or 
both). The language for expressing the meta-rules is 
very close to the elementary tree language, except 
that meta-rules use meta-variables standing for 
subtrees, lle proposes to integrate the meta-rules to 
the XTAG system which would lead to an efficient 
maintenance and extension tool. 
(Evans et al., 95) have proposed to use I)ATR 
to represent in a compact and efficient way an 
I,TAG for English, using (default) inheritance (and 
thus full trees instead of partial descriptions) and 
lexical rules to link tree structures. They argue the 
advantage of using ah:eady existing software. But 
some information is not taken into account : the 
lexical rules do not update argument index. For 
instance the dative shift rule for English changes the 
second complement - the PP - into a NP, which is 
not semantically satisfying. The passive rules simply 
discards the first complement (representing the 
canonical direct objet), the other complements 
moving up. But then the relation between the active 
object and the passive subject is lost. 
The three cited solutions give an efficient 
representation (without redundancy) of an f.TAG, 
but have in our opinion two major deficiencies. 
First these solutions use inheritance networks 
and lexical rules in a purely technical way. They give 
no principle about the form of the hierarchy or the 
lexical rules 2, whereas we believe that addressing the 
practical problem of redundancy should give the 
opportunity of formalizing the well-formedness of 
elementary trees and of tree families. 
And second, the ,wnerative aspect of these 
solutions is not developed. Certainly the lexical rules 
are proposed as a tool for generation of new 
schemata or new classes in a inheritance network. 
But the automatic triggering, ordering and bounding 
of the lexical rules is not discussed. 
4 Proposed solution : efficient 
representation and semi-automatic 
generation 
We propose a system for the writing and/or the 
updating of an \[,TAG. It comprises a principled and 
hierarchical representation of lexico-syntactic 
structures. Using this hierarchy and p,'inciples of 
well-formedness, the tool carries out all the relevant 
crossings of linguistic phenomena to generate the tree 
families. 
This solution not only addresses the problem of 
redundancy but also gives a more principle~based 
representation of an LTAG. The implementation of 
the principles gives a real generative power to the 
tool. So in a sense, our work can relate to (Kasper et 
al., 95) that describes an algorithm to translate a 
Head-driven Phrase Structure Grammar (I-\['PSG) into 
an LTAG. The inheritance hierarchy of tlPSG and its 
principles are "flattened" into a lexicalized formalism 
such as \[,TAG. The idea is to benefit from a 
principle-based formalism such as 1 IPSG and from 
computational properties of an I,TAG. 
2Becker gives a linguistic principle for the bounding of his meta-rules, but he has no solution tor the application of this 
principle. 
195 
4.1 Hierarchical representation of an 
LTAG 
4.1.1 Formal choices : a monotonic 
inheritance network, without meta-rules 
Like the solutions described in section 3, our 
system uses a multiple inheritance network. Yet, it 
does not use meta-rules. Though they could be a 
further step of factorization, it seemed interesting to 
"get the whole picture" of the grammar within the 
hierarchy, and not only the base trees. 
Further, we have chosen monotonic inheritance, 
especially as far as syntactic descriptions are 
concerned. Default inheritance does not seem to be 
justified to represent tree schemata, from the 
linguistic point of view. Default inheritance is often 
necessary to deal with exceptions. One may want to 
express generalizations despite a few more specific 
exceptions. Now the set of tree schemata we intend 
to describe hierarchically is empty of lexical 
idiosyncrasies, which are in the syntactic lexicon (cf. 
section 1). The set of tree schemata represents 
syntactic phenomena that are all productive enough 
to allow monotonicity. This resulting hierarchy will 
then be more transparent and will benefit from more 
declarativity. 
Technically, monotonicity in syntactic 
descriptions is allowed by the use of partial 
descriptions of trees (Rogers and Vijay-Shanker, 92; 
94), as was proposed in (Vijay-Shanker and 
Schabes, 92) (see section 4.1.3). 
4.1.2 General organization of the hierarchy 
Section 1 briefly described the organization of an 
LTAG in families of trees. The rules for the 
organization of a family, its coherence and 
completeness, are flattened into the different trees. 
With the approach of an automatic generation of 
TAG trees, we have found necessary to explicit these 
rules, which are defined using the notions of 
argument and syntactic function. 
Following a functional approach to 
subcategorization (see for instance Lexical 
Functional Grammar, (Bresnan, 82)), we clearly 
separate the "redistributions" of syntactic functions 
of the arguments from the different realizations of a 
given syntactic function (in canonical, extracted, 
cliticized.., position). We intend the term 
redistribution in a broad sense for manipulation of the 
number and functions of arguments. It includes cases 
of reduction of arguments (e.g. agentless passive), 
restructuration (dative-shift for English) or even 
augmentation of arguments (some causative 
constructions 3, introducing an agent whose function 
is subject). Redistribution is represented in our 
system by pairing arguments and functions, and not 
in terms of movement 
So the proposed hierarchy of syntactic 
descriptions (for the family anchored by a verb) 
comprises the three following dimensions : 
3We talk about some causative constructions analysed as 
complex predicates with co-anchors in French as in' 
Jean a fait s'assoir les enfants. *Jean made sit ihe children. 
(Jean made the children sit) 
dimension 1 : the canonical subcategorization h'ame 
This dimension defines the types of canonical 
subcategorization. Its classes contain information on 
the arguments of a predicate, their index, their 
possible categories and their canonical syntactic 
function. 
dimension 2 : the redistribution of syntactic 
functions 
This dimension defines the types of redistribution of 
functions (including the case of no redistribution at 
all). The association of a canonical subcategorization 
frame and a compatible redistribution gives an 
actual subcategorization, namely a list of argument- 
function pairs, that have to be locally realized. 
dimension 3 : the syntactic realizations of the 
functions 
It expresses the way the different syntactic functions 
are positioned at the phrase-structure level (in 
canonical position or in cliticized or extracted 
position). This last dimension is itself partitioned 
according to two parameters : the syntactic function 
and the syntactic construction. 
4.1.3 Monotonic inheritance and partial 
descriptions of trees 
The hierarchy is a strict multiple inheritance network 
whose terminal classes represent the elementary trees 
of the LTAG. These terminal classes are not written 
by hand but automatically generated following 
principles of well-formedness, either technical or 
linguistic. 
A partial description is a set of constraints that 
characterizes a set of trees. Adding information to 
the description reduces monotonically the set of 
satisfying trees. The partial descriptions of Rogers 
and Vijay-Shanker (94) 4 use three relations : left-of, 
parent and dominance (represented with a dashed 
line). A dominance link can be further specified as a 
path of length superior or equal to zero. These links 
are obviously useful to underspecify a relation 
between two nodes at a general level, that will be 
specified at an either lower or lateral level. Figure 3 
shows a partial description representing a sentence 
with a nominal subject in canonical position, giving 
no other information about possible other 
complements. The link between the S and V nodes is 
underspecified, allowing either presence or absence 
of a cliticized complement on the verb. In the case of 
a clitic, the path between the S and V nodes can be 
specified with the description of figure 4. Then, if we 
have the information that the nodes labelled 
respectively S and V of figures 3 and 4 are the same, 
the conjunction of the two descriptions is equivalent 
to the description of figure 5. 
4Vijay-Shanker & Schabes (92) have used the partial 
descriptions introduced in (Rogers & Vijay-Shanker, 92), but we 
have used the more recent version of (Rogers & Vijay-Shanker, 
94). The difference between the two verskms lies principally in 
the definition of quasi-trees, first seen as partial models of trees 
and later as distinguished sets of constraints. 
196 
S 
I 
S Vr 
N VO CI VO 
Figure 3 Figure 4 
s 
N Vr /\ 
CI VO 
Figure 5 
In the hierarchy of syntactic descriptions we 
propose, the partial description associated with a 
class is the unification of the own description of the 
class with all inherited partial descriptions. As 
shown in the above example, the conjunctkm of two 
descriptions may require statements of identity of 
nodes. Rogers and Vijay-Shanker (94) foresee, in the 
case of an application to 'FAG, the systematic 
identity of lexical anchors. Further, Vijay-Shanker 
and Schabes (92) make also use of a particular 
function to state identity of argumental nodes. But 
this is not enough as one might need to state equality 
of any type of nodes (like the S nodes in the above 
example). To achieve this in our' system, one simply 
needs to "name" both nodes in the same way. 
dimension 1 
Callonifa\[ StlbCat ffa\[llC 
// "\\ //f 
//>t... 
/ \ ... ! strict Iransitivc~ 
\ / 
dimclmion 2 
redistribution of syntactic 
ftulclions 
personal fttll 
paSSIVe 
Remember we talk about descriptions of trees. In 
these objects, nodes are referred to by constants. 
Two nodes, in two conjunct descriptions, referred to 
by the same constant are the same node, and two 
nodes referred to by different constants can either be 
equal or different. Equality of nodes can also be 
inferred, mainly using the fact that a tree node has 
only one direct parent node. 
We trove added atomic features associated with 
each constant, such as category, index, quality (i.e. 
foot, anchor or substitution node), canonical 
syntactic function and actual syntactic function. 
These features belong to the meta-formalism of 
I~TAG hierarchical organization. We will call them 
meta-features (as opposed to the features attached 
to the nodes of the TAG trees). In the conjunction of 
two descriptions, the identification of two nodes 
known to be the same (either by inference or because 
they have the same constant) requires the unification 
of such meta-features. Ira case of failure, the whole 
conjunction fails, or rather, leads to an unsatisfiable 
description. 
dimension 3 
realization of syntaclic fullclions 
subjecl par-object 
c'Inollical posillon wh-qucslioncd 
pt)Sl\[IOll 
hand-written 
hicnuchy 
I WOnOVnl-pass h genclatcd class 
(strict transitive, \] 
personal full lmssive. | 
!)!~.r-obj wh-qucstmned)~ 
Figure 6. (_'reation of a terminal class totally defil~.ed by ffs super-classes. 
4.2 Automatic generation of elementary 
trees 
The three dimensions introduced in section 4.1.2 
constitute the core hierarchy. Out of this syntactic 
database and following principles of well- 
formedness the generator creates elementary trees. 
This is a two-steps process : it first creates some 
terminal classes with inherited properties only - they 
are totally defined by their: list of super-classes. Then 
it translates these terminal classes into the relevant 
elementary tree schemata, in the XTAG 5 format, so 
that they can be used for parsing. 
The tree schemata are generated grouped in 
families. This is simply achieved by fixing a 
canonical subcat frame (dimension 1), associating 
XTAG (\[ amubek et al., 92) is a tool for writin~ and using LTAGs, including among other things a tree editor and a syntactic 
parser. 
with it all relevant redistributions (dimension 2) and 
relevant realizations of functions (dimension 3). At 
the development stage, generation can also be done 
following other criterions. For instance, one can 
generate all the passive trees, or all trees with 
extracted complements... 
4.2.1 Principles of well-formedness 
The generation of elementary trees from more 
abstract data needs the characterization of what is a 
well-formed elementary tree in the framework of 
\[,TAG. The common factor to various expressions of 
linguistic principles made for \[,TAGs is the 
argument-predicate co-occurrence principle (Kroch 
and Joshi, 85; Abeill6, 91) : the trees for a predicative 
item contain positions for all its arguments. 
But for a given predicate, we expect the 
canonical arguments to remain constant through 
redistribution of functions. The canonical subject 
197 
(argument 0) in a passive construction, even when 
unexpressed, is still an argument of the predicate. So 
the principle should be a principle of predicate- 
functions co-occurrence : the trees for a predicative 
item contain positions for all the functions of its 
actual subcategorization. In the solution we propose, 
this principle is translated as : 
1- subcat principle : a terminal class must inherit of 
a canonical subcategorization (dimension 1) and a 
compatible redistribution, including the case of no 
redistribution at all (dimension 2). This pair of 
super-classes defines an actual subcategorization. 
2- completeness/coherence/unicity principle : the 
terminal class must inherit exactly one type of 
realization for each function of the actual 
subcategorization 6. 
Well-formedness of elementary trees is also 
expressed through the form of the hierarchy itself 
(the content of the classes, the inheritance links, the 
inheritance modes for the different slots...). This 
information spread into the hierarchy is used for tree 
generation following technical principles of well- 
formedness. Due to a lack of space we detail only 
the following principle, useful to understand next 
section. 
3- unification principle : the unifications of partial 
descriptions and meta-equations required by 
inheritance must succeed; the unification of nodes 
with same constant is mandatory; moreover two 
nodes with the same value for the meta-feature 
"function" must unify. 
Figure 6 shows an example of generation of a 
terminal class, corresponding to the tree, for French, 
for the full passive of a strict transitive verb, in a 
wh-question on the agent (see figure 7). it can be 
illustrated by the sentence : 
(Je me demande) par qui Jean sera accompagn6. 
By whom will Jean be accompanied? 
Sr 
PP S 
~N0,1, NI~0 I 
par 
Figure 7. Tree for French, for the full passive of a strict transitive 
verb, in a wh-question on the agent. 
The corresponding terminal class W0n0Vnl- 
pass inherits the canonical subcat STRICT 
TRANSITIVE and the redistribution PERSONAL 
FULL PASSIVE. This defines the following actual 
subcategorization : arg0/par-object; argl/subject. 
Then the terminal class inherits the relevant 
realization for each of the cited functions (SUBJECT 
IN CANONICAL POSITION and PAR-OBJ- 
QUESTIONED). 
6Following from the functional representation of 
subcategorization, this principle relates to the principles of well- 
formedness of functional structures in LFG. 
4.2.2 From terminal classes to elementary 
trees 
The terminal classes representing elementary 
trees inherit a (constructed) partial description of 
tree, with meta-equations and equations. To get 
elementary trees from these classes, we need to 
translate the partial descriptions into trees. This is 
done by taking the least tree(s) satisfying the 
description. We do not go into the details for brevity 
reasons, but intuitively the minimal tree is computed 
by taking the underspecified links to be path of 
length zero when their ends are compatible, of length 
one otherwise (figure 8). A description can leave 
underspecified the order of some daughters, leading 
to several minimal trees. Rogers and Vijay-Shanker 
(94) give a formal mechanism to obtain trees from 
descriptions. 
s s 
NP VI' NP VI' 
' I ! ! v v 
Figure 8. Translating a dashed line into a path of length one. 
After obtaining tree(s) from the partial 
description, the generator translates the node 
constants into the concatenation of syntactic 
category and index (if it exists). 
4.2.3 A detailed example 
Let us go back to the tree of figure 7. The next 
figure shows in detail the super-classes 7 (introduced 
at figure 6) for the class W0n0Vnl-pass representing 
this tree : 
STRICT TRANSITIVE 
meta-equations : 
?arg0.canonical-function = subject 
?arg0.ind = 0 ?argl.canonical-function = 
object 
?argl,ind = 1 
PAR-OBJECT wh-questionned position 
topology :/~N, N 
?PI' ?S 
?par~"NN~ \[ ?quest 
par 
meta-equations : 
?Sr.cat = S ?parP.cat = P 
?Sr.ind - r ?SP.cat : SP 
?quest.cat : N 
?questfunction = par-obj 
f PERSONAL FULL 
PASSIVE 
topology : ?S 
?sup VO I 
I ?inf 
meta-equations : 
?S.cat = S 
?sup.cat =V ?inf.cat = V 
?inf.ind = in ?inf.qual = $ 
?arg0.function = par-obj 
~argl.function = subject 
f • 
NOMINAL SUBJECT 
canonical position 
topology : ?S 
?subject ?sup 
meta-equations : 
?subject.functkm = subject 
?s.ubject.c.at = N ,, 
Figure 9. Super-classes of W0n0Vnl-pass. 
7We only show the direct super-classes. They are given 
with their specific properties and with their inherited properties 
as well. The "equations" slot is not shown. In the partial 
descriptions shown, the constants naming the nodes start with ?. 
198 
The conjunction of the inherited partial 
descriptions leads to the following description : 
?Sr 
?PP ?S 
?parP ?quest ?subject ?sup VO 
I : par ?in~ 
Figure 10. Inherited partial description. 
The nodes with same constants have unified 
(?S/?S) and the constants with same "function" 
meta-feature have also unified : ?subject/?argl and 
?quest/?arg0 (cf. principle 3). Then the node 
constants are translated and the least satisfying tree 
is computed, leading to the target tree of figure 7. 
5 Applications 
The tool has been used to update and augment 
the French LTAG developed at Paris 7. A hierarchy 
has been written that gives a compact and 
transparent representation of the verbal families 
already existing in the grammar. The writing of the 
hierarchy has been the occasion of updating 
structures and equations, insuring uniform and 
coherent handling of phenomena. Furthermore the 
automatic generation from the hierarchy guarantees 
the well-formedness of the families, with all possible 
conjunctions of phenomena. Extra phenomena such 
as nominal subject inversion, impersonal middle 
constructions, some causative constructions or free 
order of complements have been added. 
The generative power of the tool is effective : out 
of about 90 hand-written classes, the tool generates 
730 trees for the 17 families for verbs without 
sentential complements 8, 400 of which were present 
in the pre-existing grammar. The tool is currently 
used to add trees for some elliptical coordinations. 
We see several possible applications of the tool. 
We could try to generate a grammar with weaker 
constraints, useful for corpora with recurrent ill- 
formed sentences. Secondly, we could obviously use 
the tool to build a grammar for another language, 
either from scratch or using the hierarchy designed 
for French. Using this already existing hierarchy and 
the implemented principles of well-formedness will 
lead to a grammar for another language "compatible" 
with the French grammar. This could be an 
advantage in the perspective of machine translation 
for instance. 
Because the principles of well-formedness 
implemented are general and capture mainly the 
extended domain of locality of LTAG, the generator 
we have presented can very well be used to generate 
a grammar with different underlying linguistic 
choices (for instance the GB perspective used in the 
English grammar cited). 
8 By the time of conference, we will be able to give figures for the families with sentential complements also. 
6 Conclusion 
We have presented a hierarchical and principle- 
based representation of syntactic information. It 
insures transparency and coherence in syntactic 
descriptions and allows the generation of the 
elementary trees of an LTAG, with systematic 
crossing of linguistic phenomena. 
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199 
