Kernel Metamodel documentation

* Add UML diagram
* Description of metaclasses
* Clarify that there are two constraint services

Issue-Ref: 3401
Issue-Url: https://af3-developer.fortiss.org/issues/3401



Signed-off-by: Simon Barner <barner@fortiss.org>

org.fortiss.tooling.kernel/html/developer/.ratings

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org.fortiss.tooling.kernel/html/developer/documentation.html

@@ -26,9 +26,59 @@ The kernel also provides abstract base implementations for most of the handler i
 which are located in the <code>org.fortiss.tooling.kernel.extension.base</code> package.
 </p>
 
+<h2> Metamodel description </h2>
+<h3>Tooling Kernel Metamodel Classes</h3>
+<img alt="UML diagram of Tooling Kernel" src="figures/kernel.png"/>
+
+<p>The root package of the tooling kernel metamodel consists of the following classes:</p> 
+<ul>
+<li><code>IIdLabeled</code>: Model elements implementing this interface have a unique identifier.</li>
+<li><code>INamedElement</code>: Model elements implementing this interface have a unique id and a name.</li>
+<li><code>INamedCommentedElement</code>: Model elements implementing this interface have a
+unique id, a name and a comment.</li>
+<li><code>IProjectRootElement</code>: Super class of all root elements contained in the project.</li>
+<li><code>ILibraryRootElement</code>: Super class of all root elements contained in the libraries.</li>
+<li><code>ILibrary</code>: Super class of all libraries.</li>
+<li><code>ILibraryElement</code>: Super class of all elements contained in a library.</li>
+<li><code>ILibraryPackage</code>: Super class of all library package (that can contain library
+elements and/or sub-packages)</li>
+<li><code>ILibraryElementReference</code>: Super class of all references to elements contained
+in a library.</li>
+<li><code>IExternalDocumentReference</code>: Reference to an external document.</li>
+<li><code>ISpeciallyCopyiable</code>: Interface to hook code to handle special cases for copy and paste.</li>
+</ul>
+
+<p>The <code>constraints</code> sub-packages contains the following meta-classes required for the
+<code>IConstraintService</code>-based constraint verification mechanism (see below):</p>
+<ul>
+<li><code>IConstrained</code>: Element which can be constrained. Keeps track of all the constraints
+which apply to it.</li>
+<li><code>ConstraintInstance</code>: Constraint &quot;instance&quot;: link between the constrained
+element(s) and the constraint. Explicitly represented in the model in order to store the status and the
+checksum(s) of the constrained element(s).</li>
+<li><code>IConstraintInstanceStatus</code>: Status of verification of a constraint. Typically,
+should denote &quot;Success&quot;, &quot;Fail&quot;, &quot;Outdated&quot; or &quot;Error&quot; with
+possible specializations depending on the particular constraints.</li>
+<li><code>OutdatedConstraintInstanceStatus</code>: Status that indicates that the verification
+of a constraint is outdated.</li>
+<li><code>FailedConstraintInstanceStatus</code>: Status that indicates that the verification of a
+constraint has failed.</li>
+<li><code>ErrorConstraintInstanceStatus</code>: Status that indicates that an error has occurred
+during the verification of a constraint.</li>
+<li><code>SuccessConstraintInstanceStatus</code>: Status that indicates that a constraint has
+successfully been verified.</li>
+<li><code>IConstraintInstanceContainer</code>: Container to group multiple constraint instances.</li>
+<li><code>ConstraintNameToChecksum</code>: Map type to map constraint names to checksum values.</li>
+</ul>
+
 <h2>Package Overview</h2>
+In the following, we describe the top-level packages of the Tooling Kernel plugin.
+
 <h3>constraint</h3>
-Package for constraint checkers of the kernel.
+<p>Package providing several basic <code>IConstraintChecker</code>-based constraint checkers
+(supported by <code>IConstraintCheckerService</code>).</p>
+Note that the kernel also provides <code>IConstraint</code>-based constraints (supported by
+<code>IConstraintService</code>).</p> 
 
 <h3>extension</h3>
 <p>
@@ -154,4 +204,5 @@ kernel service.
 <h3>utils</h3>
 Utility methods provided by the kernel.
 
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