A TWO-WAY APPROACH TO STRUCTURAL TRANSFER IN MT 
Rebecca Root 
Linguistic.s Research Center 
University of Texas 
P.O. Box 7247 
Austin, Texas 78712 
USA 
Abstract 
The METAL machine translation project incorporates two 
methods of structural transfer - direct transfer and transfer by 
grammar. In this paper I discuss the strengths and weaknesses 
of these two approaches in general and with respect to the 
METAL project, and argue that, for many applications, a 
combination of the two is preferable to either alone. 
1. Introduction 
One of the central design questions in machine translation is 
that of the best method of structural transfer, that is, the 
conversion from the syntactic analysis structure of the source 
language to the syntactic generation structure of the target 
language. Although several of the various approaches to this - 
interlingua, transfer grammar, and direct transfer \[Slocum, 84\] - 
share a number of properties which reader a choice among them 
of relatively little consequence, there is at least one point of 
variance that can have significant practical ramifications. This 
is the choice between the ~ of an independent grammar, as one 
finds in the interlingua and tran~er grammar approaches, and 
direct transfer, where transfer specifications are tied directly to 
source language structures. Since each method has its 
advantages and disadvantages, there is no b~-qi~ for favoring one 
over the other, independent of a particular application. 
However, it is highly likely that for a system with any significant 
range of application, neither approach will be completely 
satisfactory. Furthermore, decisions made in the design of other 
components of the system may render a homogeneous approach 
to transfer impractical. For both of these reasons, we have 
implemented in METAL a scheme for transfer which is 
sufficiently flexible to allow for the use of both direct transfer 
and transfer by grammar. This is done in such a way as to put 
control of the interaction in the hands of the grammar writer, 
allowing him to take maximum advantage of the strengths of 
each approach. 
in the following, I will contrast the strengths and weaknesses 
of the two methods mentioned above and illustrate how a 
combination can inherit the advantages of each by discu~ing our 
experiences with a combined system in METAL. For the sake of 
clarity, I will first give an overview of the METAL architecture. 
2. Overview of METAL 
METAL is a machine translation system designed for the 
translation of technical texts. Currently, it is implemented for 
German to English translation, but preliminary work has begun 
on other language pairs. These efforts indicate that, by and 
large, the design is suitable for application to multiple source 
and target languages, and work is in progress to make this 
completely so. 
Translation proceeds in three phases: analysis, integration, 
and transfer. The analysis phase consists of parsing the input 
sentence and building a phrase structure tree annotated with 
various grammatical features. Anaphoric links are resolved 
during the integration phase \[Weir, 1985\]. During the transfer 
phase, the parse tree is structurally and lexicslly modified 
according to target language specifications. The output sentence 
is gotten by reading the terminal nodes of this tree. 
Our basic method of structural transfer is a fairly direct 
transfer. Rather than using a separate transfer grammar, 
transfer instructions are associated with each rule of the analysis 
grammar. W"hen an analysis rule applies to build a node, stored 
on that node, along with grammatical features, is the set of 
transfer instructions associated with that rule. After integration, 
the selected parse tree is traversed from top to bottom, executing 
the transfer instructions associated with each node. The 
instructions typically consist of such things as feature passing, 
constituent reordering instructions, tree traversal messages, and 
lexical transfer instructions. Since the grammar writer chooses 
what transfer instructions to include and how to order them, he 
has significant control over the flow of the transfer procedure. 
An example of such a rule is given here. This is a rule for 
parsing German prepositional phrases. I have left out the 
various TEST, CONSTRuction and INTEGRation instructions 
relating to analysis and integration. See \[Bennett, 1983\] for a 
complete description of the grammar component. Comments 
explaining the English transfer instructions are given in italics. 
PP PP.EP NP 
1 2 
TEST 
CONSTR 
IlCTE~ 
~gGT.ISH 
(SEF 1 CA GC) father's CAse becomes first 
son 'e Grammatical Case 
(XFR) transfer the sons, 
i.e. descend the tree 
(Am) 
(INT 1 P0 POST) if first son hae \]:~)sition POST, 
(XFM FLIP)) make it follow the second son 
The prepoaition's value for GC is updated because this can 
resolve English transfer ambiguities. After this modification, the 
sons are transferred according to the English instructions found 
on their nodes. After transfer, the preposition, now with English 
features becanse the node has been transferred, is checked for its 
position requirements. If it is a postposition, it is placed after 
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the noun phrase. For example, the structure associated with the 
phrase "vor einer Woche" is modified to yield a structure 
reflecting the the phrase "a week ago'. When other target 
languages are included, their transfer instructions will appear in 
this rule as SPAfqlSH, CHINESE or whatever. In this way, one 
analysis could simultaneously serve as input to the transfer 
procedures for several target languages. 
The type of direct transfer described here has several good 
points. It is very efficient because ~ere is no time wasted in 
trying rules which don't apply. By the same token, it is fairly 
easy for the linguist to guarantee the results of the transfer 
process because he can gear his rules to very specific structures. 
For example, there are several Ger~,~ constructions which are 
analyzed by rules with a phrase structure specification NP -> 
NP NP. One of these is the genitive construction, as in "ein Teil 
des Programms'. The English transfer set se~ciated with this 
particular rule contains instructions to insert the English genitive 
marker "of" so that the translation becomes "~ part of the 
program'. There is no wasted attempt to make this insertion in 
the similar, but not genitive, constructions. Likewise, transfer 
procedures peculiar to thoec structures are not applied in vain to 
the genitive construction. As one might suppoae, this method 
also has the real, if somewhat embarrassing, advantage of 
allowing for fairly easy implementation of ad hoc solutions, 
which, unfortunately must be resorted to from time to time. 
There are, of course, several disadvantages to doing things 
this way. If there are multiple source languages, the linguist 
must repeat, in perhaps non-trivial ways, the same target 
language information for each aource grammar. There is no 
convenient way to sta~e more global linguistic facts that don't 
relate to immediate constituent structure (this is a problem for 
analysis as well). ALso, this method forces the description of the 
target language to be made in terms of the constituent structure 
of the source language. All of the~ are problems which are 
better handled in a grammm- based approach to structural 
transfer. Our decision to incorporate a franker grammar grew 
out of the need to overcome the last two restrictions, particularly 
in the treatment of clauses. 
3. The use of transfer grammar in METAL 
The moat pr~,~in_S need for grammar based transfer was the 
result of the adoption of a canonical clause structure. The 
original impetus for using a canonical structure wu the need for 
an efficient analy,,ds of the German clause. However, this 
canonical structure is put to use by METAL in another way, one 
which will, in all likelihood, insure its utility, or at least its 
necessity, for all source languages. The area which would require 
this is lexical transfer. 
Because the dependency between a verb and its object can 
influence greatly the lexical and structural transfer of both, as 
well as the structural transfer of the clause as a whole, it is very 
useful to do a certain amount of lexical transfer, in particular, 
verb transfer, at the clause level, where both the verb and its 
arguments are &variable for inspection and manipulation. This is 
not a new idea. What is important here is that, although the 
grammar writer determines when and how clause level lexical 
transfer takes place, the proper functioning of the transfer 
procedure depends on the canonical structure of the clause. See 
\[Bear, 1983\] for a complete description of the lexical transfer 
process. The structure we employ is a flat structure, consisting 
of a PREDicate node followed by one or more arguments: 
<clausal c;Lr, egory / \ 
/ \ 
PPXD *R~I C ..3 C ..3 ARGn 
However useful a canonical structure is for analysis and 
lexical transfer, and, in principle, for structural transfer, it 
creates problems for our direct, node by node structural transfer. 
The effect of transforming during analysis and integration is that 
the constituent structure that is reflected by the analysis rule is 
by no means the constituent structure that actually exists at 
transfer time for the node built by that rule. This can be 
illustrated by the following two trees for the sentence "dem Kind 
gab der Mann den Ball'. The first is the parse tree that would 
have been bnilt ff the tree had not been transformed. The 
second is the actual tree that is built. The circled nodes are ones 
which are eljm;nffited by flattening, the boxed node is one whose 
sons have been changed. 
S f 
/\ 
den If.t nd 
PP~D / 
VB ! 
g~b 
NP /\ 
DET NO ! ' 
DET NO d n Ball 
M~nn 
/ /\ /\ /\ 
VB DE'I" NO DET NO DET NO \[ \[ 1 I t 1 1 
g~.b dem Kind der Mznn den B~II 
Obviously, the transfer portion written for the rule giving 
the boxed node, CLS -> NP RCL, can have very little specific to 
say about the transfer proce~ becanse the actual sons and their 
order are not at all predictable from anything in this rule. The 
power to make the various examinations and permutations 
nec,~mry to execute an appropriate transfer does exist, but they 
can o-ly awkwardly be specified. Furthermore, they would 
necesasrlly be repeated throughout the grammar. The flattening 
described here takes place in the construction of all clause type 
structures, and so this same crop of sons could be found hanging 
on a wide variety of trees. Rather than forcing such a 
treatment, we exploit what is known about the canonical 
structure to reap the benefits of treating what is e~entially an 
interlingua as such, by manipulating its structure through the 
application of transfer grammar rules. This is done in the 
following way. 
Transfer rules are implemented as packages of instructions, 
typically including tree transformations, of the type found in the 
target language portion of an analysis rule. However, rather 
than being stored on a node by virtue of that node's parse 
history, they comprise an independent portion of the system and 
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are invoked by instructions in target packages. Transfer rules 
are stored according to one or more root categories. Rules 
pertaining to a particular category are invoked when the target 
package associated with a node of that category invokes ORO, 
the program which accedes the transfer grammar. Because this 
program is called directly from the grammar and under control 
of the grammar writer, the overall tran~er efficiency is not 
degraded by the use of a transfer grammar. Any additional cost 
associated with the use of this grammar is born locally by the 
constructions which directly benefit. The tran~er package 
a.~ociated with the boxed node is given here: 
CLS NP RCL 
ENGLISH 
C~.SXFR) do main verb transfer 
(ORO) invoke grammar rules 
for this category 
(XFR) descend the tree and transfer sons 
An example of one transfer rule which ORO would invoke is 
given below. The first line is a list of root categories to which 
this rule applies. Thes rule tests to see whether the clause is 
indicative, and if it is, invokes a transformation by means of the 
function XFM to place the subject NP before the main verb. 
The structural description of this transformation is met if the 
first son is of category PRED and if there is some son following 
it of category NP and having the value SUBJ for the feature 
ROL, i.e., some noun phrase fullfdls the grammatical role 
subject. The description allows for the possibility of zero or 
more constituents preceeding and/or following the NP. 
CLS-SUB LCL RCl. CLS-REL 
(AND 
(Ilcr I m3 DID SUB) 
if PREDicate is INDicative 
or SUBjunctim MooD. 
(XFM .. move SUBject in front of PREDicate 
C/E:1 (PRED:2 -:3 CNP:4 NIL (I~Q R0L SUB J)) -:5)) 
(Z:I (NP:4 PRED:2 -:3 -:S))))) 
It might well be asked whether there will be any role for 
direct transfer in a multilingual system, if it has been found to 
not be completely satisfactory in a bilingual one. I tend to think 
there will be, although the role will, no doubt, be reduced. 
There will probably always be the need for ad hoc solutions to 
isolated transfer problems, and there is no reason why s'~ch non- 
general solutions should not take advantage of the efficiency 
available by a more specific direct transfer. And at the very 
least, this method offers an excellent way to give the linguist 
control over the flow of the transfer process. The combined 
capability is particularly valuable when one considers not only 
the requlremen~ of a completed system, but those of a system 
still under development, as well. 
REFERENCES 
Bear, John. "Aspects of the Transfer Component of the 
METAL Machine Translation System," unpublished manuscript, 
1984. 
Bennett, Winfield S. "The LRC Machine Translation 
System: an Overview of the Linguistic Component of METAL," 
Computers and Artificial Intelligence, vol. 2, no. 2, April 1983. 
Slocum, Jonathan. *Machine Translation: It's History, 
Current Stattm and Future Prospects', COLING, 1984. 
Weir, Carl. "Anaphora Resolution in the METAL Machine 
Translation System," unpublished manuscript, 1985. 
There are a variety of rules for placement of other clause 
constituents. The results of the call to ORO at the clause level is 
then a tree whose major constituents reflect English word order. 
Transfer of the constituents thenmelves is then accomplished by 
descending the tree in the usual manner. 
The discumion above involves only changes which reorder 
constituents. The transfer grammar also includes rules for more 
drastic structural changes, such as placement of the particle 
"not" and the subject of questions within the English verb 
auxilliary. 
4. Summary 
We have, so far, only utilized the transfer grammar in 
places where a direct approach would lead to extreme 
redundancy in transfer with respect to one language pair. Our 
treatment of English clauses, however, a~o has the advantage of 
reducing redundancy acro~ source languages, since the 
requirements of the transfer lexicon insure that the input 
structure to these rules would remain the same. It is likely that 
further work in other language pairs will give rise to other uses 
of the transfer grammar. 
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