DIMENSIONS OF SUSTAINABILITY

Alexey Voinov
<avoinov@uvm.edu>


Courtland Smith
<smithc@ucs.orst.edu>
Oregon State University, Department of Anthropology, Corvallis OR 97310, (503) 737 3858




Abstract

No agreed definition of sustainability has emerged. As a result, people define sustainability in the ways that suit their particular applications, often times with no explicit evidence and recognition of the exact meaning being implied. We analyze several definitions, and find that while there is no common meaning, the definitions can be organized according to a common set of dimensions. Space and time are common in systems analysis and characterize all the definitions considered. Added to these are structural and perceptual dimensions. Our analysis indicates that sustainability should be treated within the framework of a total system, taking into account the ecological, social and economic as components of the system. It is impossible to sustain one part of the total system without the others being involved. It is therefore more reasonable to speak about systems sustainability instead of sustainability of resources or sustainable development (economic bias with ecological concerns), or sustainability of ecosystems (ecological bias with economic concerns) . We try to merge ecology and economy into one system coming up with the conditions for sustainability instead of defining the term in an exact way.

Key words

Systems sustainability, value sets, conditions for sustainability, stability, spatial and temporal scales. Human adaptation is deleted.

A Systems Perspective on Sustainability

What do people mean when speaking about sustainability? Sustainable yield management emerged as the dominant paradigm in the early twentieth century. The promise of sustainable yield was continuous supplies of fish, food, and fiber, without degrading natural resources. The practice of sustainable yield management has been a series of crises. The Brundtland Commission report, Our Common Future (WCED, 1987), re-energized interest in sustainability, however since that time, no common definition of the term has come forth, despite considerable literature appearing in both the scientific and public press. See for example, the special issue of Ecological Applications - Vol.3, No.4, Nov. 1993 - for an extensive discussion on various sustainability problems.

Most sustainability definitions originate from the relationship between humans and the resources they use. For example, among the 75 definitions of sustainability collected from students at Oregon State University 65% explicitly identified sustainability as resource management and use practices. Wimberly (1993:1) states that "to be sustainable is to provide for food, fiber, and other natural and social resources needed for the survival of a group -- such as a national or international society, an economic sector, or residential category -- and to provide in a manner that maintains the essential resources for present and future generations". Gale and Cordray (1991) look at eight and later (1994) at nine types of resource sustainability.

Some of the definitions are very descriptive and focus on humanitarian and ethical aspects. According to Solow (1991), "sustainability as a moral obligation is a general obligation, not a specific one." Norton (1992:25) argues that "sustainability is a relationship between dynamic human economic systems and larger, dynamic, but normally slower changing ecological systems, such that human life can continue indefinitely, human individuals can flourish, and human cultures can develop - but also a relationship in which the effects of human activities remain within bounds so as not to destroy the health and integrity of self-organizing systems that provide the environmental context for these activities". However in that same book we find the definition reversed when an ecological system is termed healthy "... if it is stable and sustainable" (Haskell, et al., 1992:9). In another definition Costanza (1992:240) leans on the systems properties, stressing that "sustainability... implies the system's ability to maintain its structure (organization) and function (vigor) over time in the face of external stress (resilience)".

Part of what makes defining sustainability controversial is that the issue actually brings ecological problems from the realm of pure science to the everyday life of people. Defining sustainability connects abstract environmental issues with people's personal and commercial interests. As a result, sustainability becomes one of those terms which is easier to understand, than to explain.

In many cases the sustainability goal is being applied only to the economic part of the development process and the ecological part is only considered as a background on the stage where economy is developing. Nearly all sustainable resource issues come from this economic prerogative, with the attempts to make the ecological resources last as long as possible in support of economic development. Many issues of sustainable agriculture also belong to this economic domain. There is a relatively smaller portion of sustainability concerns that prioritize the ecological part of the problem (Nash, 1991). We mostly find them in the green movement (Greenpeace or Earth First, as the extreme presentation of these concepts), or in theories of "deep ecology" (Devall and Sessions, 1985). In this approach the goal of sustainability is primarily imposed on natural systems, which are to "remain in pristine conditions" (Taylor, 1986).

Generally, in most of the literature, the ecological part and the economic part are considered as separate systems operating independently. Effort is made to compare the two systems (Holling et al., 1993) or to look at their coevolution, trying to combine their dynamics. Ecology is being incorporated into economy, giving rise to the new science of ecological economics (Costanza, 1991).

Still there is relatively little effort to look at the economic and ecological parts within the framework of one and the same system. In terms of sustainability this becomes especially crucial, because it is not possible to speak about long-term development of one of the subsystems without taking into account the other. It seems to be much more reasonable to think general system sustainability, realizing that the economic and ecological components are to be considered together in their interplay.

Within the systems approach it becomes clear that there are numerous feedback links between the social, biological and physical parts and that it is not possible to sustain one part without some trade-offs from the others. In their "sustainable" development over the last 12 000 years humans have usually neglected the sustainability of nature, trying to build the world in their own way, reconstructing the nature, trying to mold it to suit their needs best. The few cautioning voices were hardly heard and only after humans became a geological power (Vernadsky, 1978) did they realize that they were about to pass the point of no return, that the technocratic approach, too, has its drawbacks, and that it may not suit the aesthetics and morals of all and may not itself be sustainable. However there is great inertia in human societies. Some count on new technologies that should solve all environmental problems, others do not care, still others care and understand that something has to be done but are not ready to change their habits and life styles. The development continues and the environments change. The overall problem is that the emerging environments may be quite different from what we are used to, what we expect for human habitat, and change may be irreversible.

Do these created technocratic environments suit the needs of the majority? Can they be sustainable? With no clear answers to these questions there is the other group of definitions of sustainability, that tend to prioritize the changes within the human as part of the biosphere, the trade-offs and sacrifices that people may be willing to bring in order to keep some of the natural control mechanisms and natural cycles, instead of totally depending on the artificially imposed human management.

For this purpose it is necessary to merge all the subsystems that make up the human life support system, to look at them in their interplay and to decide how to make the whole system sustainable. Starting with a brief exploration of the resource sustainability types, we will look at the other definitions of sustainability, primarily focusing on the concept of system sustainability.

Spatial Dimensions of Sustainability

Gale and Cordray (1994) in their sociological study of various attitudes toward sustainability have come up with the following types:

Putting aside the possible critiques of this typology and its appropriateness, let us use it as an example of the variety of existing approaches to sustainability and see what are the inherent scales for the different approaches. Let us only note that actually the listed sustainability types do not present the different possible designs of sustainable systems, but rather deal with the rationale that is used to promote sustainability. This typological study immediately refers us to the varying understanding of sustainability, that corresponds to the different ideas and priorities set up by the proponents.

In systems analysis there are usually three dimensions along which a system can be considered. These are time, space, and structure. It is within these dimensions that we choose the level of accuracy and the scales in which the system is analyzed. The spatial dimension represents how the system is represented in space, what spatial components are identified, what are the spatial borders of the system and what are the links to other systems across the borders. The temporal dimension describes the level of resolution for system dynamics in time, the time step of analysis, the temporal events that should be either singled out or lumped together. And finally the structural dimension is the level of detail that is chosen to describe the processes and functions in the system, the variables and the links between them.

The different definitions of sustainability assume different scales and resolutions within the three dimensions. At a first glance the temporal scale is less informative because practically all sustainability definitions tend to depict sustainability as extending to infinity - what else would we be anticipating by this notion. The spatial dimension brings more insight. In Figure 1 we tried to arrange the nine sustainability types along the spatial axes. The available nine points are clearly not a sufficient sample for statistical analysis, however it is interesting to note that the economically dominant sustainability issues tend to the smaller, local scales: both the designed systems and the interested groups are smaller, more localized. On the contrary, there are hardly any applications that would strive for globally sustainable economic systems. Surprisingly in the global level most of the concerns are usually about the biosphere as a system with the ecological focuses and priorities rather than the economic ones. This observation leads us to the more general question about the possible spatial scales on which sustainable systems are to be designed.

Looking at how ecosystems and economies operate, we may note the importance of feedback mechanisms that control and adjust the processes in systems. One of the reasons why market based economies proved to be most efficient is because they employed a number of self-regulating mechanisms either operating at the local scale or networking together the locally operating subsystems into a higher level system with efficient feedbacks. Cooperation, but mostly competition and prices based on demand and supply constantly tuned the overall system functioning. At the same time the presumably more rational and efficient systems of planned economy failed to really account for all the cause-effect links and factors and result in constant malfunctioning of the economic mechanism. The feedbacks inherent to systems work independently of human errors and will.

However the ecological and economic components of systems operate in significantly different scales. The ecological feedbacks are generally long both in time and space. The feedbacks and in socioeconomic systems are rather short (Miller, 1992: 139). The problem with sustainability is how to make the ecological feedbacks shorter to incorporate them into the economic and social subsystems, how to mirror the regional and global environmental outcomes of our everyday life in something we can feel and evaluate locally, right on the spot, at home? An economic crisis, for instance, almost immediately affects our everyday life - prices change, income drops, etc. To close the loops of system sustainability we need to map the global environmental change in something we use and care for locally. Of course an overall price tag associated with all our activities could do the job, but unfortunately we still do not price ecological commodities in an appropriate and unambiguous way.

For general sustainable systems we need to hypothesize a spatial level at which feedback links impose self-control on the subcomponents of the system. There need to be mechanisms that directly feedback the ecological consequences of human behavior into everyday life. The environmental degradation is usually either remote in space, happening in distant localities, in other countries or continents, or relatively slow, delayed for further generations to cope with. The institutional mechanisms that are supposed to feedback such ecological responses are usually either not in place or not yet efficient enough. Until larger scale feedback mechanisms are created for the overall functioning of sustainable systems, we should try to operate at the lowest level creating small scale sustainability in the individual or family level. This calls for special efforts in educational and awareness building programs that influence and form the value sets and human behavioral patterns.

Values in Sustainability

We therefore see that another dimension lies in the way the domain of human values is structured. The first possible values dichotomy in the approaches towards sustainability is between the economic and ecological focuses, between anthropocentric and ecocentric attitudes. On the one hand we have anthropocentric definitions that assume maintenance of the human well-being in the traditional consumers society (DPS, DSS, HBS, GPS in Gale and Cordray's typology). In all these cases it is assumed that humans are the most important part of the ecosystem and ecosystem function is sustained primarily for their benefit. On the other hand we can identify the ecocentric approaches to sustainability, where the ecological well-being of the whole planet or of a certain region is emphasized regardless of the direct benefits of the human population inhabiting it (GNP, EIS, SSS, EIN or EBS).



Going further on in classification (Fig.2), we find that among the ecocentric definitions there are two structurally distinct approaches: one which may be called "true ecocentric", which is based on faith that in fact the human species is no better than the others and that the humanity should lower its ambitions and needs. The other ecocentric approach is actually an anthropocentric one, since it stresses the ecological priorities because it understands that the only strategy of sustainable survival for the human being is to limit itself to the ecologically feasible level. In the first case we make our decisions in favor of ecological benefits as a result of our faith and belief in nature and species rights, while in the second case we basically act because of our wisdom and understanding that in the long run the humans will only benefit from nature conservation and harmony, and actually choose the priority of ecological interests based on logic and some knowledge.

In turn the anthropocentric approaches may be focusing either on economic or on social values and benefits. Accordingly, the decisions are being made to maximize either the social or economic criteria. It might be argued that eventually the ecocentric and anthropocentric approaches tend to get closer merging together as we consider the long-term evolution at the global scale of the human population. In this case some of the aspects of global ecology become important from the viewpoint of human health and aesthetic benefits. However it should be also realized that for this factor to become meaningful we must assume a total revolution in human perception of its role and place in modern world as well as its existing living standards and priorities. It is not likely that general public is ready to take up these other standards immediately.

Referring back to figure 1, we add another dimension to the picture. Let us look at the system sructure, considered at increasing levels of resolution or desegregation. We may argue that there is a correspondence between the resolution or the amount of information used to describe the structure of a system and the attribute of human values, prevailing in the system. The aggregated simplistic approach with few feedbacks taken into consideration, with little understanding of the processes, and little concern about the longer term and larger scale effects in the system will probably generate a simplified anthropocentric approach with the prevailing economic set of values. The limited amount of information about the links within the system and with the rest of the world does not stimulate awareness about the possible remote outcomes of human activities, the ecological components are treated only as resources that are in place to support the human component and the major concern is the optimization of resource uptake and processing. As more details are taken into account, more information about the feedbacks is considered and the system is analyzed as a whole with more links and processes, we gradually shift to the more ecologically oriented value sets and eventually arrive at the ecocentric value set placed at the other end of the structure/value axis. As a result we may look at types of sustainability in a 3 dimensional space with the two domains in Fig.3 representing the two extreme cases: on the one hand (A) sustainability can be viewed as of a purely economic goal to serve our local momentary needs; in the other case (B) we are mostly concerned with preserving the whole biosphere for its own sake and for as long as possible. In one case we operate in terms of a simplified model, looking only at the sustainability of resources provided by the nature. In the other we draw a holistic picture in which humanity acts within the biosphere as its element.



Other definitions of sustainability fit somewhere in between. However we still must admit that it is mostly the rationale, the human logic of sustainability that differ and that we class, rather than the sustainable system design. Adding the temporal scale in this diagram we want to show that, while all sustainability definitions emphasize maintaining a particular system to infinity, still we note that the anthropocentric approaches tend to operate on lower scales, being concerned with relatively short term systems (local economies, communities, etc.).

Dynamics of Sustainability

The temporal scale for evaluating system sustainability also gives additional insight. The temporal dimension shows the dynamics of sustainable systems, how and why they change in time. It may be interesting to compare the sustainability notion with that of stability.

Instead of "sustainability" in ecology, the topic usually was "stability". Ecological stability has been discussed in extensive literature. Both general biological and rigorous mathematical definitions have been suggested. Sustainability is still lacking both.

According to the classic definition of stability we assume that there is a certain state or trajectory that the stable system returns to after being disturbed within certain limits (Fig.4). The property of stability was mostly analyzed within the framework of ecological systems, described in terms of such variables as population number, biomass, density, concentration. Most often the system was considered as operating on its own and stability was sought as its intrinsic feature. In this case the dynamics of the system were considered as self-contained with the goal of the system imbedded into it. Human social and economic activities were considered as external, causing perturbations and the major issue was whether the system was stable enough to accept and damp out these perturbations, maintaining its ecological natural origin.

The notion of sustainability comes into play when we realize that we cannot delimit the system in such a way that all the essential links, inputs and outputs can be taken into account. Pristine ecosystems, which can be separated from the human world, are hard to find (Botkin, 1990), so the natural ecological system operates in a world of human social relations, which are much more difficult to bound, predict and understand. Actually it is still just the other way round, but for particular ecosystems the effect of humans already turns out to be crucial and as humanity continuous its expansion its environmental impact becomes dominant. Most important is that the social values and as a result the goal functions, the indicators that an individual or system tries to optimize as it develops, are under constant change, because of constantly changing perceptions and priorities within the societies. Therefore in case of analyzing sustainability the goal may be imposed on the system externally. The sought state or trajectory of the system is to be changed and the question is whether the system has internal resources to respond appropriately or if there exist additional external controls that can bring the system to the newly set dynamics (Fig.5).



Robinson (1991) stresses that sustainability calls for maintenance of the dynamic capacity to respond adaptively. Instead of analyzing the system structure in order to understand its behavior and the state or dynamics that it will demonstrate, we are now investigating the flexibility of the system to change according to the changing goals and controls. With the system displaying the behavior presented by trajectory A, suppose that due to some changes in the values and social perceptions, we now want the system to follow trajectory B. If we can find the appropriate control mechanisms and change the system parameters or structure in such a way that the system will follow trajectory B, then we may call our system sustainable. It should be noted that in this case human decision makers, take the very responsibility of constantly managing the system in such a way that this, maybe unnatural, equilibrium is preserved. Sustainability appears as dynamically maintained stability.

In one of the definitions quoted above, an ecological system is called healthy "if it is stable and sustainable" (Haskell et al., 1992:9). We would argue that sustainability is a broader notion, which encompasses stability and should refer to the whole system level rather than only to the ecosystem one.

The stable state is being achieved due to some natural intrinsic mechanisms. System sustainability assumes constant management. The system will be sustainable if there exists such a management scenario, that will bring it to the desired state or dynamics. It will be unsustainable if the management scenario does not exist, or if it cannot be applied due to some external conditions (say, financial limitations). In turning to the sustainable development paradigm society in a way takes a greater responsibility. Sustainability is always defined within the particular domain in the values set. That is, the property of sustainability is dependent upon the human requirements imposed on the system.

If we could create a socioecological model that would take into account not only the ecosystem biological and physical processes, but would also describe the social and economic relations as functions of the other system components, then probably we could go back to analyzing system stability. In the sustainability analysis we admit that there is so far no understanding of how and why decisions are made, what actually drives human priorities and values. We are not yet ready to incorporate these factors in a formalized fashion for further analysis within one single model. Therefore we are analyzing system controllability instead of stability to provide for the changing social values and priorities. Or as Salwasser (1993:587) puts it: "sustainable development is a moving target. It has multiple dimensions, scientific, economic, and political, many of which are not amenable to scientific illumination".

This brings us back to the notion of a common socioecological system for analysis. In order to achieve the sustainable dynamics we have to match the desirable behavior produced by the social values system with what is possible on the ecological part. There are always certain limits to the adaptability of the ecological component and it should not be overstrained. In a certain way this can be considered as a dynamic stability, which is being achieved by both managing the ecological subsystem and molding the social goals in an adaptive way.

The social component that produces the goals is also a function of the environment in many cases to a much greater extend than we even realize. In our approach to sustainability analysis we adopt the idea of a close link between the social systems and their carrying environments, as it has been suggested by Gumilev (1989) in application to the development of ethnoses in conjuncture with their carrying landscapes. We observe worldwide that there is a correlation between the state of the environment and the socioeconomic status of the region. The higher economically developed nations usually have higher environmental standards and concerns. However, we may also observe sustainable environments in the pristine areas with very low living standards and primitive economies. The most important prerequisite of sustainability is the balance between the social desires and ecological capacities. This brings us finally to the following attempt to describe sustainability in terms of a set of value conditions.

Conditions for Sustainability

Attempts to find an exhaustive definition for sustainability seem to be quite futile - there are too many nuances sprouting from the particular applications and implementations of the term. In such a situation it might be more productive if instead of giving precise definitions, we follow the axiomatic approach and focus on describing the conditions that the system is to comply with to achieve sustainability. The axiomatic approach is widely used in abstract sciences such as mathematics or logic and result in many productive applications, even though at first sight it may seem that this leads to cyclic definitions. Deciding about the necessary conditions for sustainability instead of defining it, may serve as a basis for building consensus between various interested parties, in a way making clear the common ideas that sustainability necessary implies.

Such necessary conditions may be formulated as follows:

1. the system does not cause harm to other systems, both in space and time;
2. the system maintains living standards at a level that does not cause physical discomfort or social discontent to the human component;
3. within the system life-support ecological components are maintained at levels of current conditions, or better.

We may also call this a perceptual definition because it is based on a series of terms which we do not know how and do not intend to define. At the same time their meaning is usually intuitively implied, when considered in some particular instances. We do not specify what is "harm" or "better", or what does "discomfort" or "discontent" mean, however in each application we may assume a procedure to measure these indicators.

The first condition is quite obvious if we want to look at a system in the higher scale as part of another system. If the "bad things" are exported outside the system, the sustainability of the system can not be considered, because then we automatically have to assume that something else "bad" can be imported from outside. This is related to an important assumption we have to make when delimiting the system. We want the parties involved to admit that everything they do may be also done by the other interacting parties locally and/or globally.

In the temporal dimension the currently existing systems may feel quite well protected from the effects of the systems that are to replace them in the future. It is the present generation that affects the future ones. Therefore in this dimension the condition becomes primarily a moral issue that leads to studies of intergenerational equity and justice (Costanza, Daly 1992, Golley, 1994, Glasser et al., 1994). Spatially the systems have more obvious bi-directional feedbacks and respond to mismanagement in quite remote locations.

This condition is strongly related to educational and traditional backgrounds that identify the value sets to be applied. Some of the actions are much easier to exclude by a taboo-type rules, rather than by common sense and logical reasoning. In a way this relates the goals of sustainability to some of the fields addressed by faith and religion. In each particular case the issue of harm can be different. It very much depends on the existing trade-offs between different subsystems and their mutual willingness to cope with the inconveniences created by different close and remote neighbors. The only fact that a certain subsystem generates toxic waste of a certain type does not make it totally unbearable by the surrounding subsystems. There may be forms of compensation that a polluting system can offer, either in form of economic benefits or serving as sink for other types of pollution in exchange. In any case this problem is supposed to be resolved in a manner of mutual agreement and fair play.

It is therefore important to identify the borders of the system to be considered. For instance when speaking about sustainable agriculture, we may look at a field or a farm as the system to be taken care of. With adequate agricultural practices and sufficient investments in the soil rehabilitation procedures we may attain long-lasting sufficiently high yields. However analyzing the sustainability of such a system we are to take into account the sources of, say, energy used, fertilizers and manure applied, etc. In this way it may easily turn out that the energy has been produced in a non-sustainable manner, or the production of fertilizers was generating pollution, and so on. It becomes a question then whether the particular agricultural field in this case can be considered a sustainable system. Similar examples may be found in larger and smaller scales. After choosing the system borders the next step is to trace all the inputs and outputs and analyze the pathways of information and material coming into and leaving the system. This first principle may be helpful in setting certain restrictions within the systems, so that local borders can be defined, otherwise we shall inevitably end up at the global scale.

The second condition for sustainability is also quite obvious if we want the system to be sustained. If the living conditions in a certain region no longer suit the people living there, it results in social tension that eventually reorganizes the social and economic components of the system. This occurs in form of gradual depopulation (emigration or die- off), or sharp conflict (wars, revolutions), that changes both the numbers and the system structure. In any case the system is to change its initial design and therefore is to fail the sustainability test. Important is the mode of change. In sustainable systems the change takes place as a result of actions accepted by the society and not causing conflict (discomfort, discontent). The same events occurring against the societal choice make the system unsustainable.

This can be also considered in the perceptual framework. The living standards and living patterns are very much a function of local traditions and education. The living standards that would seem luxurious for a family with little wealth would turn out to be insufficient for somebody who is already well- off. These differences become further pronounced when comparing across different cultures and traditions. The living standards that seem acceptable and even desirable in some countries would hardly suit people in other parts of the world. In many cases the best solution for attaining sustainability is to decrease material desires by shifting priorities in people's value systems. We are therefore inclined to be looking not at an absolute indicator of life quality in terms of GNP or market price, but as we formulated above at the content and comfort of the inhabitants.

A corollary that follows from this condition is that it is hardly possible to achieve sustainability in the developing or transient economies in such regions as Africa or Former Soviet Union. Economic transitions assume wide shifts in social and political institutions, they become possible as a result of discontent and rejection of the status quo by the majority of the population. This is a stage of reconstruction, which can be hardly associated with sustainability.

The third condition parallels the second, but for the biotic, ecological component of the system. It is therefore important to identify the signals of the ecosystem that would communicate its "content" or "comfort" for the given state. This implies a set of some overall ecological indicators or indices. The intensively developed concept of ecosystem health (Costanza et al., 1992) may be useful at this stage. In this way the ecosystem health notion is used to identify system sustainability, rather than the sustainability concept applied for identifying the ecosystem health. The current conditions of the system are an important factor in deciding whether the system should be sustained or not. On the other hand the property of system sustainability depends upon the current conditions of the system. For certain conditions sustainability may not be achieved. This again parallels sustainability with stability, when stable dynamics was a function of the initial conditions of the system. In case of sustainability there is more flexibility in achieving the desired state due to the external controls at our disposal -- theoretically we may always assume new technologies or methods that would solve all the problems and bring the system to the desired regime. However in practice there are always limits (defined by the available resources or time).

The perceptual look at sustainability produces a different set of indicators. While speaking about resource sustainability the indicators that are usually suggested tend to be measurable and continuous. The kind of axiomatic definition given above results in indicators of a discrete type: yes or no, either the system complies with the listed conditions or not. In this case we do not want to compare systems with respect to their sustainability, to define which system is more sustainable, which is less. We do not assume intermediate stages: either the system is sustainable and then it satisfies all the restrictions, or it is not sustainable. We may note that these conditions for sustainability merge the ecological and economic into one system. The three conditions meet the criteria of dealing with the temporal, spatial, and structural dimensions common to sustainability definitions. They may be applied only in the level of general system dynamics. Therefore sustainability becomes a general systems property, when all the variety of system components are taken into account. To coin a term for such sustainable systems, we may call them sustems, and try to promote sustems analysis.

Acknowledgments

The research has been supported by a grant from the Center for Analysis of Environmental Change, Oregon State University. We thank Richard Gale and Jim Wigington for stimulating discussions.

References



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