# Introduction

An architecture context is a method for customizing Papyrus for a given domain. It defines a regime for editing a Papyrus model semantically and notationally. The semantic editing is defined by a set of element type set configurations (from some domain), while the notational side is defined by a set of viewpoints consisting of a set of representation kinds (e.g., diagram kinds and table kinds) and other notational configurations.

An architecture context is composed within an architecture domain (a namespace) and satisfies a set of concerns that belong to a set of stakeholders, both of which are composed in (the same or another) architecture domain. An architecture domain can be specified by one or more architecture models. In the latter case, all of these models are dynamically merged at runtime.

An architecture context can either be an architecture description language or an architecture framework. In the former case, it can own its own set of representation kinds, while in the latter, it cannot. However, in both cases, a context can own a set of viewpoints that reference representation kinds owned by any description language.

Each model in Papyrus is created using a specific architecture context, although this context may later be switched, possibly resulting in the model details being changed. A model also specifies a subset of its context's viewpoints as enabled at creation time, although this set may later be switched as well. When a viewpoint is enabled on a model, a user can create representations, based on this viewpoint's referenced representation kinds, and see them in model explorer. When a viewpoint is disabled, its representations become invisible, unless another enabled viewpoint references their representation kinds.

# Architecture Context Preference Page

Papyrus is typically deployed with one or more architecture models that are registered through platform extensions. The architecture contexts contributed by these models can be seen in the Architecture Context Preference Page:

A user can choose to enable or disable those architecture contexts in the workbench by toggling their checkboxes, set one of them as a default context (using the Make Default button), or extend existing contexts or add to them at runtime by specifying other architecture models from the file system or the workspace (using the Other Architecture Models button):

# Architecture Context UI Elements

The enabling of architecture contexts in a workbench has some impacts on the following UI elements in Papyrus:

## Select Architecture Context

The Select Architecture Context page of the New Papyrus Model wizard allows for selecting one of the enabled architecture contexts to create the model in. It also allows for enabling/disabling on the model the set of viewpoints that are provided by the context.

## Initialization Information

The Initialization Information in the New Papyrus Model wizard has a Select a Representation Kind section that is populated with only the diagrams and tables that are available in the enabled viewpoints.

## Switch Architecture Context and Viewpoints

The Switch Architecture Context... context menu action in the Model Explorer view allows for switching the model's architecture context and/or viewpoints. Similarly, the Switch Architecture Viewpoints context menu allows for quickly enabling/disabling a viewpoint.

### Switch Architecture Viewpoint

When the Switch Architecture Viewpoints is disabling viewpoint(s), there are these behaviors:

- The diagrams/tables provided by this viewpoint(s):
  - are closed (if they are not enabled by another viewpoint)
  - are still visible in Model Explorer, Welcome Page, Hyperlink and Table of views but with a grayed icon
  - cannot be opened (double click on them opens a window to activate the correct viewpoint)
- The new diagram/table menu for the diagrams/tables in this viewpoint(s) becomes not available

On the contrary, when the Switch Architecture Viewpoints is enabling viewpoint(s):

- The diagrams/table provided by this viewpoint(s) are visible with initial icon in Model Explorer, Welcome Page, Hyperlink and Table of views
- The new diagram/table menu for the diagrams/tables in this viewpoint(s) becomes available

## Unsupported Diagrams/Tables filter

The Unsupported Diagrams/Tables filter in the Model Explorer view hides diagrams/tables whose kinds are not supported by the viewpoints currently enabled on the model.

## New Diagram/Table menu

The New Diagram and New Table contextual menus for the model elements in the Model Explorer view provides only the diagrams and tables that are available in the enabled viewpoints and are applicable to the currently selected model element. For example, when a UML Activity is selected, the New Diagram context menu will not offer to create a Package diagram, or a class diagram. The same holds for the toolbar elements for the creation of diagrams and tables.

## Properties view

The diagrams properties show the following information:

## Diagrams/Tables

- Diagram Type or Table Kind is the qualified name of the representation kind. It gives an indication of which architecture context is providing it.
- The Owner attribute is the model element that syntactically contains the representation in the model explorer. It can be different from the root element.
- The Root element attribute is the model element that is the representation's root semantic element.

# Definition of New Architecture Models

Papyrus supports the definition of new architecture models that can subsequently be used by selecting them as other models in the Papyrus Architecture Context preference page, as presented above. Papyrus architecture models are defined in files with the *.architecture extension. They are really just EMF models that can be edited with the specialized architecture model editor provided by Papyrus.

## Basic Concepts

The architecture metamodel in Papyrus is implemented as a realization and an extension to the ISO 42010 standard for architecture of systems. Hence many concepts present in the metamodel are derived from those presented in the ISO 42010 standard. However, the standard has been extended with Papyrus-specific concepts and properties.

- A domain is the root of the architecture model. It can contains a collection of concerns, stakeholders, as well as contexts (which can be description languages and/or frameworks).
- A concern (from ISO 42010) represents in Papyrus a matter of interest to some stakeholder.
- A stakeholder (from ISO 42010) represents in Papyrus an archetype of users. It references a set of its concerns.
- A description language (from ISO 42010) represents in Papyrus a modeling language (e.g., UML, SysML). It has a unique identifier that corresponds to that of an IClientContext from GMF. It references a metamodel, a set of profiles, a set of element type set configurations, a creation command (creates a model of this language), a conversion command (converts a model to this language), and an icon. It also has an optional extension prefix for its models. It can also contain a set of representation kinds (which can be either diagrams or tables) and a set of viewpoints. This element can also provide TreeViewerConfigurations elements to configure the display of the model in TreeViewer (used in Dialogs and in the Model Explorer).
- A framework (from ISO 42010) represents in Papyrus a modeling framework (e.g., DoDAF). It has a unique identifier that corresponds to that of an IClientContext from GMF. It references a set of element type set configurations, a creation command (creates a model of this framework), a conversion command (converts a model to this framework), and an icon. It also has an optional extension prefix for its models. It can also contain a set of viewpoints.
- A viewpoint (from ISO 42010) in Papyrus references a set of representation kinds, which can be diagrams or tables.
- A diagram in this context does not represent a single instance (for example the diagram named X in model Y), but the specification (or prototype) of all diagrams of this kind. For example the UML Class Diagram.
- A table is another kind of view in Papyrus that enables the presentation of models in a tabular format.
- A tree viewer configuration is used to configure the display of the model in TreeViewer (used in Dialogs and in the Model Explorer). An implementation of it is provided by the class EMFFacetTreeViewerConfiguration

## Extension and Inheritance

Papyrus adds to the ISO 42010 standard two mechanisms that enable reuse of and additions to architecture contexts provided by other bundles. These are extension and inheritance. Extension allows an architecture context to add definitions to other contexts; these added definitions are merged as appropriate (see below) with definitions already in the receiving context and contributed to it by other extensions. Inheritance allows an architecture context to reuse definitions from another context, integrating them into itself in accordance with the same merge semantics as extension: inherited definitions are merged into the inheriting context's local definitions and also any definitions that may be added to it by extensions.

### Context Extension

A common use case for context extension is the addition of custom viewpoints and representation kinds to some context. For example, consider a plug-in that defines a new kind of diagram or table for some peculiar visualization of elements in UML models. Papyrus provides an ''architecture description language'' covering the standard UML diagrams. An custom architecture description language can have an extension relationship to UML by adding the UML language to its extended contexts list. Its custom viewpoint with its custom representation kinds are then added to the viewpoints and representation kinds defined in the UML context.

An architecture context that extends other contexts via this mechanism is an extension context and is not instantiable in its own right in Papyrus models. Its only purpose is to contribute definitions to the contexts that it extends. In Papyrus, its definitions are merged into the contexts that it extends and then it is elided from the set of contexts available in the UI.

### Context Inheritance

Another use case involves specialization of another architecture context for domain-specific semantics. For example, UML is engineered to enable refinement of the language in various ways for the description of domain-specific modeling languages (DSMLs) such as UML-RT or SysML. For these purposes, Papyrus makes it possible to define an architecture context that inherits another context. A domain-specific architecture description language can, for example, set the UML architecture description language as its general context. This defines a new language that has all of the definitions provided by its general context (here, UML) and adds further definitions of its own. Unlike the extension scenario, a specialization context that inherits some other context is a first-class context that is available in Papyrus for creation of models in that language (or framework).

### Architecture Context Merge

In cases of extension and inheritance of contexts, definitions from one context are added to another context, albeit in different directions. This contribution of definitions from one context to another is not strictly additive: definitions that would have the same name in the same scope (thus would have equal qualified names) are merged. When Papyrus initializes its architecture definition subsystem, it loads all of the source architecture models and merges their contexts according to the semantics described below. The contexts resulting from this merge operation are what are presented in the New Model Wizard and other UI.

#### Semantics of Merge

Architecture Contexts

- in the case of inheritance, the definitions (viewpoints, representation kinds, etc.) in the general context are merged into the specific (inheriting) context. A context may inherit zero or one contexts of the same kind ( architecture description language or architecture framework)
- in the case of extension, the definitions in the extending (or extension) context are merged into each of the extended contexts. A context may extend zero or more contexts of the same kind ( architecture description language or architecture framework)
- the specific context or extended context into which some context is merged will copy attributes from the merged context as follows:
  - the receiving context copies the name of the merged context if it does not already have a name specified
  - the receiving context copies the description of the merged context if it does not already have a description specified
  - the receiving context copies the identifier of the merged context if it does not already have an identifier specified
  - the receiving context copies the icon of the merged context if it does not already have an icon specified
  - the receiving context copies the creation command class of the merged context if it does not already have a class specified
  - the receiving context copies the conversion command class of the merged context if it does not already have a class specified
  - the receiving context adds any element type set configurations of the merged context that it does not already reference
  - the receiving context merges the viewpoints of the merged context (details below)
  - the receiving architecture description language copies the referenced metamodel of the merged language if it does not already reference some metamodel
  - the receiving architecture description language adds any referenced profiles of the merged language that it does not already reference
  - the receiving architecture description language copies tree viewer configurations of the merged language
  - the receiving architecture description language merges the representation kinds of the merged language

Viewpoints

- a viewpoint received into an architecture context is merged with a viewpoint of the same name, if any, already defined in the receiving context. Otherwise, the viewpoint is just copied into the receiving context
- in the case of a merge,
  - the receiving viewpoint copies the description of the merged viewpoint if it does not already have a description specified
  - the receiving viewpoint copies the identifier of the merged viewpoint if it does not already have an identifier specified
  - the receiving viewpoint copies the icon of the merged viewpoint if it does not already have an icon specified
  - the receiving viewpoint adds any concerns of the merged viewpoint that it does not already reference
  - the receiving viewpoint merges the representation kinds of the merged viewpoint

Representation Kinds

- a representation kind received into an architecture context is merged with a representation kind of the same name and type (diagram, table, etc.), if any, already defined in the receiving context. Otherwise, the representation kind is just copied into the receiving context
- a representation kind received into a viewpoint is merged in the architecture context featuring that viewpoint, which is its namespace
- in the case of a merge,
  - the receiving representation kind copies the description of the merged representation kind if it does not already have a description specified
  - the receiving representation kind copies the identifier of the merged representation kind if it does not already have an identifier specified
  - the receiving representation kind copies the icon of the merged representation kind if it does not already have an icon specified
  - the receiving representation kind copies the grayed icon of the merged representation kind if it does not already have an icon specified
  - the receiving representation kind adds any concerns of the merged representation kind that it does not already reference
  - the receiving diagram merges the parent diagram of the merged diagram, if any
  - all other details of the merged representation kind are dropped

#### Interaction of Context Inheritance and Extension

An architecture context may participate in both inheritance and extension relationships. In this case, the context is an extension context and as such is elided by the merge algorithm.

Conceptually, the merge algorithm proceeds in two stages: first a top-down merge of contexts in inheritance relationships and second a bottom-up merge of contexts in extension relationships. This has several consequences:

- contexts do not inherit definitions that are added to their general contexts (recursively) by extensions, because inheritance is merged first. This is practically useful because it means that the inheriting context knows extactly what definitions it is inheriting: only those defined locally in its general context (and that context's general context, recursively)
- extension contexts do contribute definitions to their extended contexts that they inherit from their general context. Because inheritance is merged first, those inherited definitions are present in the specific context to be merged into the extended contexts
- contexts that inherit definitions from a general context may receive definitions from contexts that extend them

As inheritance is transitive, so is extension: a context that merges its definitions into some extended contexts will include in that merge any definitions that are merged into it by other contexts that extend it.

There are configurations of inheritance and extension that are not permitted and are flagged as errors by validation of architecture models:

- a context may not inherit itself, either directly or indirectly. Inheritance cycles are forbidden
- a context may not extend itself, either directly or indirectly. Extension cycles are forbidden
- a context may not inherit, either directly or indirectly, a context that it extends, either directly or indirectly. Otherwise, it would inherit its own definitions

The architecture contexts that result from this merge algorithm do not have any references to inherited and/or extended contexts. This reflects semantics of merge as copying definitions from merged to contexts to receiving contexts, so that these relationships are not needed in the merge result. But also, more practically, extension contexts are not included in the merge result (as they only exist to donate definitions to other contexts) and so do not exist to reference as general or extended contexts.

> For advanced tools that need it, Papyrus does provide a mechanism by which contexts and other elements in the merge result can be traced to the elements in the source architecture models from which they are derived. Those source models retain the original inheritance and extension relationships.

#### Architecture Context Debugging

In case the Papyrus workbench shows an unexpected configuration of architecture contexts, Eclipse tracing options are available to assist in the analysis of how the source architecture models were merged to produce that outcome. These are:

- `org.eclipse.papyrus.infra.architecture/debug/models` to trace the discovery and processing of source models by the Architecture Domain Manager
- `org.eclipse.papyrus.infra.architecture/debug/merge` to trace the merge algorithm as it transforms those source models into the merged architecture contexts

## Walkthrough

The definition of an architecture model in Papyrus starts with the selection of the Architecture Model option in the Eclipse New creation wizard under the Papyrus category and clicking Finish. At each following step, it is strongly recommended to validate the Architecture Model by selecting the 'Papyrus Developer'> 'Validate architecture' menu.

Step 0 (optional): If a profile is used to define the Architecture description language, prepare the profile plugin. The profile plugin shall have org.eclipse.papyrus.infra.architecture as dependencies. The profile shall extend org.eclipse.papyrus.infra.architecture.models with the path to the architecture model file as shown below.

Do not forget to include the profile and architecture resource directories in the Binary build.

Step 1: Specify the Name, Id and Description of the Architecture Domain (the root element).

Step 2: Right click on the domain to add one or more Concerns. In the properties view, specify the following for each concern: a Name, an Id and a Description.

Step 3: Right click on the domain to add one or more Stakeholders. In the properties view, specify the following for each stakeholder: a Name, an Id', a Description'', and one or more references to Concerns.

Step 4: Right click on the domain to add one or more Architecture Description Languages. In the properties view, specify for each language: a Name, a Description, a unique Id (e.g., org.eclipse.uml2.UML), an Extension Prefix if desired (e.g., profile), an Icon using a platform plugin URI (e.g., platform:/plugin/project/icons/xxx.png), a Metamodel as a reference to an EPackage (load the Ecore model first), one or more Profiles as references to EPackages (load the Ecore models first), one or more Element Types as references to ElementTypeSetConfigurations (load the *.elementtypesetconfiguration models first), a creation command using a fully qualified of a Java class implementing the IModelCreationCommand interface, and an optional conversion command using a fully qualified of a Java class implementing the IModelConversionCommand interface.

More precisely, as far as one or several profiles are concerned, launch Papyrus in a runtime and edit the plugin to add the related profile(s) to the description language as shown below.

Add, in the `CreateMyProfileTestModelCommand.java` source file, one Java Class named CreateMyProfileTestModelCommand that extends ModelCreationCommandBase as shown below.

The following code shall then be entered, provided the profile project name (here `my_profile_test`) and the profile's pathmap (here `"pathmap://toy/my_profile_test.profile.uml"`) that is the one filled in the profile plugin ( org.eclipse.papyrus.uml.extensionpoints.UMLprofile extension point).

    package my_profile_test;
    import org.eclipse.emf.common.util.URI;
    import org.eclipse.emf.ecore.EObject;
    import org.eclipse.papyrus.infra.architecture.commands.IModelCreationCommand;
    import org.eclipse.papyrus.uml.diagram.common.commands.ModelCreationCommandBase;
    import org.eclipse.papyrus.uml.tools.utils.PackageUtil;
    import org.eclipse.uml2.uml.Package;
    import org.eclipse.uml2.uml.Profile;
    import org.eclipse.uml2.uml.UMLFactory;
    public class CreateMyProfileTestModelCommand extends ModelCreationCommandBase implements IModelCreationCommand {
     @Override
     protected EObject createRootElement() {
       return UMLFactory.eINSTANCE.createModel();
     }
     @Override
     protected void initializeModel(EObject owner) {
       super.initializeModel(owner);
       Profile myProfileTest = (Profile) PackageUtil.loadPackage(URI.createURI("pathmap://toy/my_profile_test.profile.uml"), owner.eResource().getResourceSet());
       if (myProfileTest != null) {
         PackageUtil.applyProfile((Package) owner, myProfileTest, true);
       }
     }
    }

Last but not least, the Creation Command Class shall be entered in the Architecture Model properties as shown below.

Step 4A: Right click on the description language to add one or more representation kinds, i.e., Papyrus Diagram, Papyrus Table or Papyrus Sync Table (see details below).

Step 5: Right click on the domain to add one or more Architecture Framework. In the properties view, specify for each framework: a Name, a Description, a unique Id (e.g., org.eclipse.dodaf), an Extension Prefix if desired (e.g., profile), an Icon using a platform plugin URI (e.g., platform:/plugin/project/icons/xxx.png), and one or more Element Types as references to ElementTypeSetConfigurations (load the *.elementtypesetconfiguration models first).

Step 6: Right click on the description language (step 4) or framework (step 5) to add one or more Architecture Viewpoints. In the properties view, specify for each viewpoint: a Name, a Description, a unique Id (e.g., org.eclipse.uml.design), one ore more references to Concerns, and one or more references to Representation Kinds.

Step 7: Right click on the description language (step 4), to add one or more EMFFacetTreeViewerConfiguration. In the properties view, specify : a Description and the extended EMFFacetTreeViewerConfiguration or keep the value to null.

Step 7A: Right click on the EMFFacetTreeViewerConfiguration and create one or more CustomizatioReference. In the properties view, specify the Referenced Customization. You need to get the project providing the Customization in your workspace. To be able to reference the Customization, you need to load in from the workspace in your architecture file'' ( Right click -> Load Resource -> From Workspace)

Step 7B: Right click on the CustomizatioReference and create an application rule ( Absolute Order or Relative Order or Redefinition). Absolute Order allows to give the order of application (the smallest order is the first applied). Relative Order allows to define if your CustomizationReference must be applied Before or After another one. Redefinition allows to replace the a CustomizationReference with a new one.

Step 8: Deploy the model. The new architecture file can be deployed in an Eclipse plugin and registered through an extension point. The extension point to use is `org.eclipse.papyrus.infra.architecture.models`. Alternatively, the model can be deployed using the Other Architecture Model button in the Architecture Contexts preference page.

Note: If the Architecture Model you are developing is considered an extension of another model that is available in the target platform, and you want to reference some of its dependencies (like elementtypeconfigurations and palleteconfiguration, etc.) you first need to load the plugin that contains the model in your workspace. Then, using the Architecture Model editor, load the other Architecture Model using the Load Resource->Browse Workspace context menu action). This will import it using its "platform:/resource" URI. On the other hand, if the extended architecture model is rather deployed in the current platform, then use the Loaded Resource->Browse Registered Architectures context menu action. This will load the model using its "platform:/plugin" URI. Once the extended model is loaded in the editor, you can invoke the context menu action Resolve All to load all its related resources.

## Papyrus Diagram Specification

Once a Papyrus Diagram is created, in the properties view, specify for it:

- A Name (required) that is the user-visible name of the diagram. It will appear in the creation menus and property panel.
- An Icon (required), as an URI of the form `platform:/plugin/...`.
- An Implementation ID (required) which selects the physical (hard-coded) diagram in Papyrus that will be used as a base. The possible values for this property are summarized in the following table.
- An optional Parent that specifies a parent viewpoint-defined diagram to inherit from. Essentially, the inheriting diagram will defer to its parent�s rules (see below) when its own are not sufficient to take a decision.
- An optional Custom Style as an URI of the form `platform:/plugin/...`. It must point to a CSS file that will then be automatically applied to the diagram.
- An optional Palette as an URI. It must point a palette definition in XML that will be automatically applied to the diagram.
- Other attributes are inherited from the ISO 42010 implementation and are currently not used in Papyrus.

Implementation ID | Description
--- | ---
PapyrusUMLActivityDiagram | UML Activity Diagram
PapyrusUMLClassDiagram | UML Class Diagram
PapyrusUMLCommunicationDiagram | UML Communication Diagram
PapyrusUMLComponentDiagram | UML Component Diagram
CompositeStructure | UML Composite Diagram
PapyrusUMLDeploymentDiagram | UML Deployment Diagram
PapyrusUMLProfileDiagram | UML Profile Diagram
PapyrusUMLSequenceDiagram | UML Sequence Diagram
PapyrusUMLStateMachineDiagram | UML State Machine Diagram
PapyrusUMLTimingDiagram | UML Timing Diagram
UseCase | UML Use Case Diagram
PapyrusUMLInteractionOverviewDiagram | UML Interaction Overview Diagram
BlockDefinition | SysML Block Definition Diagram
InternalBlock | SysML Internal Block Diagram
Parametric | SysML Parametric Diagram
RequirementDiagram | SysML Requirements Diagram

Once a diagram has been created it can be constrained using rules (some of them are required, others are optional). There are four kinds of rules:

- Model rules constrain the type of the (root) model elements that can be visualized through this view. Hence model rules control what model elements can be selected for the Root element property of the diagrams, as shown in the capture below.
- Owning rules constrain the type of the model elements that can own the diagram itself. In practice this materializes as the type of model elements under which the diagrams can appear in the model explorer. Owning rules control what model elements can be selected for the Owner property of the diagrams, as shown in the capture below.
- Child rules constrain the type of the model elements that can be dropped within this diagram.
- Palette rules constrain the display of the diagram's palette elements.

Each rule has a Permit property that specify whether the rule authorizes or forbids the action it represents. Otherwise, the properties of the rules are as follow:

- Model Rules
  - Element (required) represents the type of the model elements to apply the rule on.
  - Multiplicity (required, default is -1) represents the maximum number of this kind of diagram that can be created for the referenced model element. -1 represents an unbounded number.
  - Stereotypes represents the set of stereotypes that must be applied in the model element for the rule to match. The stereotypes can be picked from the classifiers of the Profiles defined in the parent diagram.
- Owning Rules
  - Element (required) represents the type of the model elements to apply the rule on.
  - Multiplicity (required, default is -1) represents the maximum number of this kind of diagram that can be created for the referenced model element. -1 represents an unbounded number.
  - Stereotypes represents the set of stereotypes that must be applied in the model element for the rule to match. The stereotypes can be picked from the classifiers of the Profiles defined in the parent diagram.
- Child Rules
  - Element (required) represents the type of the model elements begin dropped.
  - Stereotypes represents the set of stereotypes that must be applied in the model element for the rule to match. The stereotypes can be picked from the classifiers of the Profiles defined in the parent diagram.
  - Origin (required) represents the type of the model elements that are the target of the drop. It is usually one of the type defined in the Model Rules.
  - Additionally, Child Rules can be completed with children called Path Elements using the New Child contextual menu. Path Elements defines a path of properties that must be used from the Origin to insert the new Element in the model.
- Palette Rules
  - Element (required) represents a pattern to match for the identifier of a palette element.
    - If the value ends with '*', it will match any palette element with the prefix specified before the '*'.
    - If the value is '*', it will match any palette element.

The minimal required rules for a diagram specification to work are:

- A model rule that allows the diagram to be created for a model element. For example, a model rule with Element set to UML Package, Multiplicity to -1 and Permit to true; meaning this diagram can be created for UML Packages and an unlimited number of diagrams can be created for each UML Package.
- An owning rule that allows the diagram to be owned by a model element (appear under an element in the model explorer). For example, an owning rule with Element set to UML Package, Multiplicity set to -1 and Permit to true; meaning an unlimited number of diagrams of this kind can be placed under each UML Package element in the model explorer.
- A child rule that allows the dropping of any element in the diagram, expressed with the Permit attribute set to true and other attributes left empty.

## EMFFacetTreeViewerConfiguration specifications

- An ArchitectureDescriptionLanguage can contains 0, 1 or more EMFFacetTreeViewerConfiguration.
- As the declaration of ArchitectureDescriptionLanguage can be split on several architecture file and merged by the framework, the TreeViewerConfiguration can also be split on several architecture files and merged by the framework. Here we explain the rules for the definition of EMFFacetTreeViewerConfiguration.
  - At least one of the EMFFacetTreeViewerConfiguration must not extend another EMFFacetTreeViewerConfiguration, to be used as starting point of the Customization application. We call these configurations roots in the next sentences.
  - Several EMFFacetTreeViewerConfiguration can extends a given EMFFacetTreeViewerConfiguration.
  - All the CustomizationReference owned by roots EMFFacetTreeViewerConfiguration must use the AbsoluteOrder.
  - The reference''EMFFacetTreeViewerConfiguration#extends'' must target an EMFFacetTreeViewerConfiguration defined in the same ArchitectureDescriptionLanguage (but not necessary in the same architecture file).
  - All AbsoluteOrder#ordervalue must be unique for a given ArchitectureDescriptionLanguage.
  - Redefinition is allowed only one time for a CustomizationReference, so you can't redefines several time the same CustomizationReference.
  - Relative Order allowed to define only one CustomizationReference before another one, and only one CustomizationReference after another one. So you can't declare several CustomizationReference before/after an other one.

### Description of the merge process

The merge is done by the class org.eclipse.papyrus.emf.facet.architecture.customizationconfiguration.comparators.CustomizationReferenceMerger. Here, the description of the process:

1. the process starts from the roots ( EMFFacetTreeViewerConfiguration which extend nobody),
2. all CustomizationReference of the roots are taken into account and sorted by order,
3. the EMFFacetTreeViewerConfiguration#extends link is used recursively and reversed.
   1. so the EMFFacetTreeViewerConfiguration extending the roots ones are applied first,
   2. then the EMFFacetTreeViewerConfiguration extending the previous ones and so one.
      1. at each step, the CustomizationReference are applied to build a intermediate result list of the CustomizationReference to apply,
         1. the EMFFacetTreeViewerConfiguration declared as redefinition are applied first,
         2. then the EMFFacetTreeViewerConfiguration declared with a Relative Location ( Before/After).

If an error occurred during this process, the error is logged and Papyrus will use the produced intermediate result list which is just a partial merge of the Customization to apply.