The Incremental Generation of Passive Sentences* 
Bernd Abb, Michael Herweg, Kai Lebeth 
Universit~it Hamburg 
FB Informatik 
AB Wissens- und Sprachverarbeitung 
Bodenstedtstr. 16 
D-2000 Hamburg 50 
Germany 
emafl: 
herweg@rz.informatik.uni-hamburg.dbp.de 
{ abb, lebeth } @informatik.uni-hamburg.de 
Abstract 
This paper sketches some basic features of the 
SYNPHONICS account of the computational 
modelling of incremental language production 
with the example of the generation of passive 
sentences. The SYNPHONICS approach aims at 
linking psycholinguistic insights into the nature 
of the human natural language production 
process with well-established assumptions in 
theoretical and computational linguistics 
concerning the representation and processing of 
grammatical knowledge. We differentiate between 
two possible kinds of stimuli within the 
generation process that trigger the formation of 
passive sentences: a Formulator-external stimu- 
lus and a Formulator-internal one. The 
Formulator-external stimulus is determined by 
the conceptual/contextual condition of agent 
backgrounding: An agentless semantic 
representation is verbalized by way of 
constructing an ergativized verbal complex in the 
morphological structure-building component, 
rather than by mapping the semantic 
representation directly onto a passive lemma. 
The Formulator-internal stimulus is an effect of 
the constraints of rapid, incremental utterance 
production; in particular, it causes the Formulat- 
* The research reported in this paper is carried out in the 
research project "Sprachproduktion: von konzeptueller 
Struktur und Kontext zur prosodischen Realisierung der 
Bedeutung" at the University of Hamburg. The project is 
funded by the German Science Foundation (DFG) under grant 
no. Ha 1237/4-1. 
or to integrate a thematically underspecified 
increment in a prominent structural environment. 
In this case, the formation of passives is a matter 
of an additional constraint on the Lemma 
Selection process: Lemma Selection is 
constrained by the structural representation of the 
utterance produced so far. 
1 Computational Modelling 
Incremental Language Production 
of 
This paper sketches some basic features of the 
SYNPHONICS account of the computational modelling 
of incremental language production with the example of 
the generation of passive sentences. The SYNPHONICS 
("Syntactic and Phonological Realization of Incremen- 
tally Generated Conceptual Structures") approach, which 
subscribes to a cognitive science perspective on lan- 
guage processing, aims at linking psycholinguistic 
insights into the nature of the human natural language 
production process with well-established assumptions in 
theoretical and computational linguistics concerning the 
representation and processing of grammatical knowl- 
edge. 
Research in psycholinguistics (e.g., Garrett 1988, 
Levelt 1989) has revealed that the process of converting 
a preverbal, conceptual content into overt speech is per- 
formed by a number of autonomous sub-processes spe- 
cialized for different tasks within the overall process: the 
pre-linguistic conceptualization component (the Con- 
ceptualizer, in Levelt's terms) plans a content to be 
expressed and delivers a corresponding conceptual repre- 
sentation to the linguistic formulation component (the 
Formulator), which in turn selects the appropriate items 
(lemmas and lexemes) from the system's lexical data 
base and, guided by the syntactic and phonological spec- 
ifications of the lexical items, produces abstract syntac- 
3 
tic and phonological representations. The output of the 
Formulator is taken up by the articulation component 
(the Articulator), whose task it is to produce a physical 
speech signal. These components are considered to be 
autonomous modules, whose modes of operation are 
each governed by theft own sets of principles and restric- 
tions. Furthermore, the system as a whole is constrained 
by there being a unidirectional flow of information, i.e., 
there is no feedback between sub-processes. 1 Finally, it 
is widely accepted that human language production pro- 
ceeds in an incremental, piecemeal fashion (Kempen & 
Hoenkamp 1987): Rather than having to wait for com- 
plete input structures, components are able to process 
fragmentary input ("increments"). As soon as a particu- 
lar component has passed the results on to its successor 
component, it is ready for processing the next input 
increment. Thus, a given increment is processed sequen- 
tially by different components, whereas components 
may operate in parallel on different increments. 
Theoretical linguists of various persuasions converge on 
the idea that a large amount of grammatical information 
that former theories of grammar handled by extensive 
rule systems ought to be captured by detailed grammati- 
cal specifications of lexical items instead. From this 
angle, the grammar of a language merely consists of a 
small set of general licensing principles for structm~ 
projected from the lexicon. The present paper subscribes 
to this view. More specifically, the SYNPHONICS 
Formulator uses a grammar for German in the mold of 
Head-driven Phrase Structure Grammar (HPSG; Pollard 
& Sag 1987, 1992). The HPSG-style lexical approach 
to basic aspects of grammar tallies with the central role 
that recent psycholingnistic theories of language produc- 
tion assign to the lexicon in the formation of linguistic 
st~ctures (lexicon-&iven generation; e.g., Levelt 1989). 
In contrast to other approaches to the computational 
modelling of empirically substantiated features of 
human language production, such as Kempen & 
Hoenkamp's (1987) Incremental Procedural Grammar 
and de Smedt's (1990) Incremental Parallel Formulator, 
however, the SYNPHONICS process model distinguishes 
strictly between declarative grammatical knowledge and 
its procedural application, thus taking the stance of 
theoretical linguistics and related computational 
approaches. As in HPSG, the declarative grammatical 
knowledge of the SYNPHONICS Formulator is repre- 
seated in a unification-based formalism with sorted lea- 
1 In contrast, AI models that are concerned with 
incremental processing (e.g., Reithinger 1992) often make 
extensive use of feedback at the cost of economy and 
cognitive adequacy. 
ture structures. Unlike deduction-based approaches to 
natural language generation in computational linguistics 
(e.g., Shieber et al. 1990), however, the SYNPHONICS 
approach involves a detailed and transparent process 
model, with sub-processes being explicitly specified at 
any point in the overall process. This property serves to 
make the model adjustable to empirical results about the 
course of human language production and open to a veri- 
fication of its claims, namely to aim at the computa- 
tional modelling of cognitive processes. 
In order to make the above considerations more concrete, 
we will discuss the roles of the Conceptualizer and the 
Formulator in the production of a particular linguistic 
construction in some more detail in the remainder of 
this paper. The discussion of the principles guiding the 
production of passive sentences serves to illustrate to 
what extent the determinants of this construction can be 
traced to the feedback-free interplay between the 
Conceptualizer and the Formulator and the constraints 
specific to the involved modules. We cannot go into the 
details of the passive here; rather, we will confine the 
presentation to some quite simple cases. In order to cap- 
ture the full range of the passive construction across 
languages, the account presented here needs to be 
enlarged in parts. 
2 The SYNPHONICS Conceptualizer 
The conceptual input into the Formulator - in short: CS 
for "conceptual structure" - is represented in the 
RefO/RetN format (Habel 1982, 1986a/b; Eschenbach et 
al. 1989). The basic representational units are Referen- 
tial Objects (ReIDs), which are stored and processed in a 
net-like structure, a Referential Net (REIN). RefOs are 
labeled, inter alia, with sortal attributes and property and 
relation designations. The notion of RefOs comprises 
the entire range of discourse entities, such as objects, 
times, and situations (events, processes, states). 
The input representation reflects certain aspects of the 
organization of the information which the 
Conceptn_ali~er delivers to the Formulator. One impor- 
tant dimension of organization is the relative promi- 
nence of conceptual units such as particular RefOs. In 
the incremental process of forming a conceptual repre- 
sentation of the content to be expressed in an utterance, 
relative prominence can manifest itself in the time 
course of conceptualization, with more prominent units 
tending towards earlier conceptualization than less 
prominent ones. The prominence of a ReID can, for 
example, be due to its perceptual saliency (cf. Flores 
d'Areais 1987), its conceptual accessibility (i.e., the ease 
with which it can be reuieved from memory: cf. Bock & 
Warren 1985) or its sortal properties (such as animacy, 
4 
humanness, etc.; eL Bock et al. 1992). We assume that 
the Conceptualizer's output representation is a stream - 
formally: a list - of RefO/RefN fragments; the position 
on the fist indicates the order of conceptualization, 
which in turn is the order in which these fragments 
("increments") are made available to the Formulator for 
syntactic and phonological encoding. 
Furthermore, we assume coherence among conceptual 
increments. This means, in technical terms of formal 
representation, that RefOs are linked by certain means, 
most notably by what we call embedding information. 
Embedding information is one instance of a RefO's con- 
nection with its conceptual environment. As an exam- 
ple, embedding information characterizes a RefO's the- 
matic role in event types and other sorts of situations to 
varying degrees of specification. 
3 The SYNPHONICS Formulator 
The SYNPHONICS Formulator, which is a formulation 
component for German sentence production, consists of 
three sub-components: the semantic encoder, which 
transforms the conceptual input structure CS into an 
abstract semantic representation SR (cf. Bierwisch & 
Schreuder 1992); the syntactic encoder, which, on the 
basis of SR, selects lexical items and forms an abstract 
syntactic representation; the phonological encoder, 
which forms an abstract phonological representation. 2 
Figure 1 (next page) shows the internal structure of the 
SYNPHONICS Formulator. 
Syntactic structures are constructed incrementally, using 
two types of SR information. At the semantics-syntax 
interface, the so-called Scheme Selector employs the 
(possibly underspeeified) embedding information associ- 
ated with RefOs in order to select abstract X-bar- 
schemata in the form of minimally specified HPSG-like 
feature structures, such as a complementation scheme, 
which reflects a functor-argument relation, or an adjunc- 
tion scheme, which reflects a modifier-modified relation. 
Thereby, the top-down construction of syntactic struc- 
ture is triggered. At the semantics-lexicon interface, the 
so-called Lemma Selector uses the sortal attributes and 
property or relation specifications of RefOs in order to 
select the appropriate lexical items, whose syntactic 
specifications serve as the starting point for the bottom- 
up projection of phrasal structures. 
Both top-down information and bottom-up information 
pass through the so-called Functional Inspector, where 
they are checked for the requirements of functional com- 
pleteness of lexical items with regard to their semantic 
demands. These concern, for example, determiners and 
case-marking prepositions as well as passive auxiliaries. 
If necessary, the Functional Inspector initiates a further 
consultation of the lexicon. 
Each newly formed syntactic structure must be licensed 
by a set of HPSG-style declarative grammatical princi- 
ples. In the case of lexical bottom-up information, the 
principles mainly effect phrasal feature projection (Head 
Feature Principle, Subcategorization Principle, 
Semantics Principle). As regards the top-down struc- 
tures, the principles serve to enrich the structural infor- 
mation specified so far (Immediate Dominance 
Schemata, Subcategorization Principle, etc.). 
Next, the so-called Integrator lakes the floor, which con- 
structs a dynamic syntactic, phonological and semantic 
representation underlying the utterance segment cur- 
rently being generated. The construction proceeds incre- 
mentally and monotonous; the only operation available 
to the Integrator is unification of feature structures 3. 
The procedural execution of integration is guided by a 
number of heuristics that reflect the need to meet the 
demands of rapid utterance production. One important 
heuristic principle crucial to the present topic is the fol- 
lowing: "Integrate phonologically filled material as soon 
as possible into the highest and lefimost position avail- 
able in the current utterance fragment." The newly 
formed increment representation is again subject to the 
grammatical licensing principles. 
4 Morphology and Syntax of the 
Passive 
Before we proceed to the application of our process 
model to the production of passive sentences, we will 
sketch the basic features of the present SYNPHONICS 
account of the syntax of the passive. 
The traditional HPSG-account of the passive (Pollard & 
Sag 1987) consists in a lexical rule that simply restruc- 
tures the elements on the SUBCAT list of a verb. The 
application of the lexical rule to the basic active entry of 
2 We will not deal with the semantic and phonological 
encoder in the present paper; cf. Gfinther et al. (1993), Abb 
et al. (1993) and the relevant papers in Herweg (ed., 1992). 
3 In order to capture nonmonotonic processes (as in the 
case of repairs or restarts), the formalism underlying the 
SYNPHONICS approach must be extended; cf. de Smedt's 
(1991) operation of non-destructive unification. 
Semantic Encoder 
Semantic RepresentaUon 
SSP- 
SCHEMES Scheme Selector 
SSP- Scheme 
top-down 
information 
Dynamic 
SSP- 
Structures 
bottom-up 
information 
I~ Fun~onal Inspector 14~1 
I " 
IH 
I Structure Ucenser i'~r'~ 
I Phonolo~lical Encoder 
£EEEND 
dedarative component 
~'1 procedural component 
Q data structure 
SSP = SemanticJSyntacticJPhonological 
Figl~e 1: The SYNPHONICS Formulator 
6 
a verb leads to a revised SUBCAT list in which the 
formerly highest NP, i.e., the subject, may occupy the 
lowest oblique position, while the former direct object 
NP takes the subject position. The initial account has 
since been modified repeatedly; we simply mention T. 
Kiss' proposal for German, 4 according to which the 
passive rule is split into two parts, a rule of Subject 
Demotion and a Subject Condition, which roughly cor- 
responds to a rule of Object Promotion. 
Rather than merely stipulating lexical rules such as 
Subject Demotion and Object Promotion, the 
SYNPHONICS account traces the effects these rules are 
intended to capture to properties of the argument struc- 
tures of the passive participle and the passive auxiliary. 5 
The morphological operation of passive participle for- 
marion gives rise to what might be regarded as an "erga- 
rivization" of the verb, i.e., the verb's external argument 
(in the sense of Williams 1983) is exempt from any 
syntactic principle that refers to subcategorized-for 
arguments. (Technically, this is realized by transferring 
the argument, which is marked by a special externality 
feature, from the verb's SUBCAT list to a blocked- 
argument \[BLOCKED_ARG\] list. 6) The passive auxil- 
iary is treated as an argument-attraction verb (cf. 
Hinrichs & Nakazawa 1991): It subcategorizes for a 
passive participle and attracts the arguments that the par- 
ticiple subcategorizes for as its own subcategorized-for 
arguments. Argument attraction is a mechanism that 
affects only the argument structure of the governed verb, 
but does not affect the primary link between semantic 
roles and arguments. The resulting SUBCAT list of the 
verbal complex is subject to the relevant grammatical 
principles, as usual. In the case of the German passive 
auxiliary werden, which we treat as an ergarive raising 
verb, the blocked external argument of the participle 
cannot be attracted. Rather, the corresponding parameter 
in the semantics will be existentially bound (if there is 
4 as yet unpublished work at IBM Germany, Institute for 
Knowledge-based Systems, Stuttgart 
5 Note that this approach is intended to capture the 
formation of the passive in morphologically rich 
languages such as German and Dutch, where passivization 
is essentially a morphological process. A different 
parametrical variation, such as the development of the 
auxiliary into a syntactic category in English, may lead to a 
passive construction that requires an analysis in syntactic 
terms. 
6 The term "blocked argument" is borrowed from Haider 
(1984), who, however, introduced it in a different 
framework. 
no oblique agent phrase). Figure 2 shows the resulting 
structure of the German participle-auxiliary complex 
gebissen werd- ("oe bitten'). 
LrB~ T ~IP ~ BLOCKED-ARG:\[~\] 
I PHON: ",aerd-" 1 l /stmc^T,m,l~ J sv-sc r, . L.  oo , 
Figure 2: Structural Descriotion of a ParticiDle- 
Auxiliarv Comnlex 
On this basis, the effects of Object Promotion follow 
from the Subcategorization Principle and a structural 
case theory that replaces the original lexical case theory 
of Pollard & Sag (1987, 1992; eL Pollard 1991). 
Technically, arguments in the lexical entry of a verb are 
marked by a case-type feature. The Subcategorization 
Principle handles the arguments of the verbal complex 
in the usual way. The new Case Principle either realizes 
the structural case type by a nominative or accusative 
value (in languages such as German and English), or 
checks for the instantiation of the values for the lexical 
case type. Due to our structural case theory, we reject an 
isomorphic relation between the order of dements on 
the SUBCAT list and the so-called hierarchy of gram- 
matical functions f'Lxed in the lexicon. Rather, we def'me 
grammatical functions, quite GB-like, in structural 
terms. From this angle, the order of elements on the 
SUBCAT list is nothing but a lexically fixed default 
prominence order of arguments. If the first argument on 
the basic SUBCAT list of a verb has been blocked, i.e., 
relegated to the BLOCKED_ARG list, the first subeate- 
gorized-for argument has to be integrated in the highest 
accessible structural position, were it receives nomina- 
tive case by means of the Case Principle. 
Figure 3 shows the structural description of the German 
passive sentence (daft) Peter gebissen wird ('(that) Peter 
is bitten'). T is the category of the functional element 
Tense; V/T is a finite verbal projection. 
V/T PHON: "Peter gebissen wird" 1 SUBCAT< > 
NP \] 
PHON: "peter" 
SUBCAT<> 
TYPE: structural 
CASE: LVALUE: nominative 
V/T 
V 
VPHON: "gebissen werd-" l 
suBcA' <  
BL(X~KED-ARG: IT\] 
\[~HON: "gebis sen 
UBCAT~/PD wird" I 
I T I PHON: 'present tense' 1 SUBCAT<\[~\] • V \[SUBCAT:\[~\]\]> 
Fimire 3: Structural Description of a Passive Sentence 
We note in passing that the theory makes the correct 
predictions for German impersonal passives, i.e., pas- 
sives without nominatively marked NPs, such as Hier 
wird getanzt \[~ere be (3 sg) danced'\],Den M~mnern wird 
geholfen \['the men (dat pl) be (3 sg) helped ~\] and Der 
Opfer wird gedacht \['the victims (gen pl) be (3 sg) 
remembered'\]. Since the passive auxiliary attracts all 
(non-blocked) argument NPs of the participle, imper- 
sonal passives are automatically formed if the partici- 
ple's SUBCAT list is empty (as in the case of getanzt) 
or contains argument NPs with lexically marked case 
only (as in the case of geholfen and gedacht). In the lat- 
ter case, the argument NPs keep their lexically marked 
morphological form. Impersonal passives lack subjects 
simply because the least oblique argument NP cannot be 
structurally case-marked as nominative. 
5 The Production of Passives 
We differentiate between two types of stimuli that trig- 
ger the production of passive sentences. The fh-St is a 
stimulus external to the linguistic system; the second is 
a stimulus internal to the linguistic system. The two 
types exemplify different ways in which the relevant 
cognitive modules - the Conceptualizer and the 
Formulator - are synchronized in order to jointly per- 
form the task of producing an utterance. 
The first case can be traced to a condition concerning the 
content of the conceptual structure CS that the 
Conceptualizer delivers to the Formulator. CS may 
include a situation-type concept (e.g., an event-type 
concept) that is marked for an agentive thematic role, 
without at the same time including the corresponding 
agentive RefO. In terms of its underlying cognitive 
function, this is an extreme case of what has been 
described as "agent backgrounding" in the typological 
literature (e.g., Foley & van Valin 1985 and Keenan 
1985). There are various motivations for agent back- 
grounding; the most notable ones are the following: 
there is a particularly salient or easily inferable agent 
(e.g., Frank Rijkaard was sent off for knocking down 
his opponent); the agent is unknown (e.g., My car has 
been stolen); the situation-type predicate alone is 
focused, with a corresponding defocusing of the agent 
(e.g., German impersonal passives of the sort Heute 
abend wird bier getanzt \['there will be a dance here 
tonight'; literally: "tonight is here danced"\]). The pas- 
sive formation device allows the Formulator to follow 
the Conceptualizer's decision to dispense with the agent 
ReiD. Thus, the two modules' principles of information 
processing tally with each other. 
More concretely, we assume that the production process 
involves the following crucial steps: The Conceptualizer 
delivers a situation type increment whose agent role 
remains unspecified (or has as yet not been specified). 
The Lemma Selector chooses an item that matches the 
corresponding semantic representation. Since this is a 
situational relation lacking its first argument, the par- 
ticiple form of the lemma, whose category is adjectival, 
is selected. The Functional Inspector completes the cat- 
egorial requirements of the situation type increment, 
which actually calls for a verbal category, by initiating a 
call to the appropriate auxiliary (i.e., werden). The com- 
plex form gives rise to a verbal projection whose exter- 
nal argument appears on the BLOCKED_ARG list and 
therefore is not subject to the Subcategorization 
Principle. The corresponding parameter is existentially 
bound in the semantics. Thus, the verbal projection sat- 
isfies the grammatical licensing conditions for construc- 
tions with non-subcategorized-for external arguments. 
The second case can be traced to a processing strategy 
that the Formulator employs when it has to react to the 
Conceptualizer's selection of a particular Reid as the 
most prominent referential CS constituent, especially 
under the constraints of rapid utterance production. In 
general, as soon as one process component delivers an 
informational increment to its successor component, the 
latter strives for further processing the increment with- 
out delay. As was claimed above, a certain Reid may be 
the first increment that the Conceptualizer passes to the 
Formulator due to its being the most prominent concep- 
tual unit in the CS selected for verbalization. Now, a 
prominent ReID argument may often be made available 
to the Formulator although its embedding information, 
such as information about its thematic role in an event 
type, is unspecified or at least underdetermined. In par- 
ticular, the ReiD may be passed to the Formulator prior 
to the situation-type concept to which it is an argument. 
In such cases, the Formulator follows the strategy to 
assign to the syntactic phrase that verbalizes the Reid 
the most prominent available position in the current 
utterance segment - i.e., in general, the structural sub- 
ject position- without waiting for information about the 
RefO's thematic role. 7 However, if it turns out sub- 
sequenfly that the phrase, due to information about the 
thematic role of the corresponding Reid available later 
on, doe~ not show the regular argument properties of 
subjects, principles guiding the Lemma Selection pro- 
cess force the formation of a passive sentence. 
In this case, the production process involves the follow- 
ing crucial steps: The ReiD increment is passed from 
the Conceptualizer to the Formulator prior to the situa- 
tion type increment. Following the above mentioned 
integration heuristic, the Integrator inserts the phrase 
corresponding to the Reid into the most prominent syn- 
tactic position, where it receives the nominative case by 
the structural Case Principle. No specific information 
about the RefO's thematic role has been used so far. At 
some stage in the process, however, such information 
must become available to the Formulator. We assume 
that this occurs when the situation type increment enters 
the Formulator. Lemma Selection is restricted not only 
by the corresponding SR, but also by information 
linked to constituents already represented in the tempo- 
rary utterance fragment constructed so far. In the present 
case, the Formulator is equipped with additional embed- 
ding information. It may turn out that the Reid whose 
realizing phrase has already been integrated in the most 
prominent position has the standard properties of an 
internal argument, for example, because it is the theme 
7 Abstracting from time factors, the subject position can, 
in more general terms, be filled without paying attention to 
the thematic role the ReID in question holds in a situation. 
This is essentially suggested by experimental studies using 
a picture-description task, where the presentation of the 
depiction of an isolated object accompanies the 
presentation of the depiction of the entire scene involving 
the object (see, e.g., Turner & Rormnetveit 1968). The 
additional presentation of the patient object, which raises 
its prominence in memory, often sets off the test subjects 
on the passive voice. See also the study reported in 
Tannenbanm & Williams (1968), in which drawing the 
speaker's attention to either the agent or the patient in a 
situation by mesns of verbal cues, thereby manipulating 
the speaker's memory access, also affected the choice of 
verbal voice. 
9 
in an actional event type. (Technically, the relevant 
embedding information is available via coindexing of 
the semantics of the already integrated NP and the theme 
argument of the situation type increment.) Lemma 
Selection must take this information into account by 
choosing a lemma with a theme as the highest subcate- 
gorized-for argument (i.e., as the final argument to be 
projected by the Subeategorization Principle). This is 
exactly the property of the participle form of the lemma 
appropriate to the situation type increment in question. 
From here on, the process of passive sentence formation 
proceeds as in the ftrst case. 
6 Conclusions 
This is by far no complete account of the determinants 
of the production of passive sentences. Rather, the aim 
of the foregoing discussion was to sketch a computa- 
tional model of natural language production that links 
psycholinguistically established aspects of linguistic 
performance with a competence model in the form of an 
HPSG-style declarative grammatical knowledge base. 
The crucial features of the process model are its incre- 
mentality and highly constrained modularity. Each sub- 
component of the overall process is governed by its own 
set of principles, with no feedback between components. 
We dealt with the relation between the pre-linguistic 
conceptualizing component and the linguistic formula- 
tion component in some detail in the present paper. The 
Concep01_~lizer's mode of operation is guided by general 
cognitive principles that, for example, select among the 
ingredients of a situation those considered to be apt for 
linguistic presentation and determine the order in which 
units of the content to be expressed are passed on to the 
linguistic component, in fact independent in principle of 
requirements specific to the latter. The Formulator has a 
number of language-system internal devices at its com- 
mand to cope with the material delivered by the 
Conceptualizer. The discussion of the production of pas- 
sives served to illustrate how the SYNPHONICS genera- 
tion system models this situation. 
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