5 files changed, 427 insertions, 274 deletions

diff --git a/src/core/model/Document.cpp b/src/core/model/Document.cpp
index a2ba5cf..4e101fc 100644
--- a/src/core/model/Document.cpp
+++ b/src/core/model/Document.cpp
@@ -135,6 +135,15 @@ bool DocumentEntity::doValidate(Logger &logger) const
 			continue;
 		}
 
+		std::unordered_set<StructuredClass *> childClasses;
+		{
+			NodeVector<StructuredClass> tmp =
+			    fieldDescs[f]->getChildrenWithSubclasses();
+			for (auto s : tmp) {
+				childClasses.insert(s.get());
+			}
+		}
+
 		// we can do a faster check if this field is empty.
 		if (fields[f].size() == 0) {
 			// if this field is optional, an empty field is valid anyways.
@@ -145,7 +154,7 @@ bool DocumentEntity::doValidate(Logger &logger) const
 			 * if it is not optional we have to check if zero is a valid
 			 * cardinality.
 			 */
-			for (auto childClass : fieldDescs[f]->getChildren()) {
+			for (auto childClass : childClasses) {
 				const size_t min =
 				    childClass->getCardinality().asCardinality().min();
 				if (min > 0) {
@@ -163,15 +172,10 @@ bool DocumentEntity::doValidate(Logger &logger) const
 			continue;
 		}
 
-		// create a set of allowed classes identified by their unique id.
-		std::set<ManagedUid> childClasses;
-		for (auto &childClass : fieldDescs[f]->getChildren()) {
-			childClasses.insert(childClass->getUid());
-		}
 		// store the actual numbers of children for each child class in a map
-		std::map<ManagedUid, unsigned int> nums;
+		std::unordered_map<StructuredClass *, unsigned int> nums;
 
-		// iterate over every actual child of this DocumentEntity
+		// iterate over every actual child of this field
 		for (auto child : fields[f]) {
 			// check if the parent reference is correct.
 			if (child->getParent() != subInst) {
@@ -195,25 +199,11 @@ bool DocumentEntity::doValidate(Logger &logger) const
 			}
 			// otherwise this is a StructuredEntity
 			Handle<StructuredEntity> c = child.cast<StructuredEntity>();
+			StructuredClass *classPtr =
+			    c->getDescriptor().cast<StructuredClass>().get();
 
-			ManagedUid id = c->getDescriptor()->getUid();
 			// check if its class is allowed.
-			bool allowed = childClasses.find(id) != childClasses.end();
-			/*
-			 * if it is not allowed directly, we have to check if the class is a
-			 * child of a permitted class.
-			 */
-			if (!allowed) {
-				for (auto childClass : fieldDescs[f]->getChildren()) {
-					if (c->getDescriptor()
-					        .cast<StructuredClass>()
-					        ->isSubclassOf(childClass)) {
-						allowed = true;
-						id = childClass->getUid();
-					}
-				}
-			}
-			if (!allowed) {
+			if (childClasses.find(classPtr) == childClasses.end()) {
 				logger.error(
 				    std::string("An instance of \"") +
 				        c->getDescriptor()->getName() +
@@ -224,18 +214,24 @@ bool DocumentEntity::doValidate(Logger &logger) const
 				valid = false;
 				continue;
 			}
-			// note the number of occurences.
-			const auto &n = nums.find(id);
-			if (n != nums.end()) {
-				n->second++;
-			} else {
-				nums.emplace(id, 1);
+			// note the number of occurences for this class and all
+			// superclasses, because a subclass instance should count for
+			// superclasses as well.
+			while (classPtr != nullptr &&
+			       childClasses.find(classPtr) != childClasses.end()) {
+				const auto &n = nums.find(classPtr);
+				if (n != nums.end()) {
+					n->second++;
+				} else {
+					nums.emplace(classPtr, 1);
+				}
+				classPtr = classPtr->getSuperclass().get();
 			}
 		}
 
 		// now check if the cardinalities are right.
-		for (auto childClass : fieldDescs[f]->getChildren()) {
-			const auto &n = nums.find(childClass->getUid());
+		for (auto childClass : childClasses) {
+			const auto &n = nums.find(childClass);
 			unsigned int num = 0;
 			if (n != nums.end()) {
 				num = n->second;
diff --git a/src/core/model/Domain.cpp b/src/core/model/Domain.cpp
index 8288099..ae20068 100644
--- a/src/core/model/Domain.cpp
+++ b/src/core/model/Domain.cpp
@@ -27,6 +27,201 @@
 
 namespace ousia {
 
+/* Helper Functions */
+
+struct PathState {
+	std::shared_ptr<PathState> pred;
+	Node *node;
+	size_t length;
+
+	PathState(std::shared_ptr<PathState> pred, Node *node)
+	    : pred(pred), node(node)
+	{
+		if (pred == nullptr) {
+			length = 1;
+		} else {
+			length = pred->length + 1;
+		}
+	}
+};
+
+static void constructPath(std::shared_ptr<PathState> state,
+                          NodeVector<Node> &vec)
+{
+	if (state->pred != nullptr) {
+		constructPath(state->pred, vec);
+	}
+	vec.push_back(state->node);
+}
+
+static NodeVector<Node> pathTo(const Node *start, Logger &logger,
+                               Handle<Node> target, bool &success)
+{
+	success = false;
+	// shortest path.
+	NodeVector<Node> shortest;
+	// state queue for breadth-first search.
+	std::queue<std::shared_ptr<PathState>> states;
+	if (start->isa(&RttiTypes::Descriptor)) {
+		const Descriptor *desc = static_cast<const Descriptor *>(start);
+		// initially put every field descriptor on the queue.
+		NodeVector<FieldDescriptor> fields = desc->getFieldDescriptors();
+
+		for (auto fd : fields) {
+			if (fd == target) {
+				// if we have found the target directly, return without search.
+				success = true;
+				return shortest;
+			}
+			if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
+				states.push(std::make_shared<PathState>(nullptr, fd.get()));
+			}
+		}
+	} else {
+		const FieldDescriptor *field =
+		    static_cast<const FieldDescriptor *>(start);
+		// initially put every child and its subclasses to the queue.
+		for (auto c : field->getChildrenWithSubclasses()) {
+			// if we have found the target directly, return without search.
+			if (c == target) {
+				success = true;
+				return shortest;
+			}
+			if (c->isTransparent()) {
+				states.push(std::make_shared<PathState>(nullptr, c.get()));
+			}
+		}
+	}
+	// set of visited nodes.
+	std::unordered_set<const Node *> visited;
+	while (!states.empty()) {
+		std::shared_ptr<PathState> current = states.front();
+		states.pop();
+		// do not proceed if this node was already visited.
+		if (!visited.insert(current->node).second) {
+			continue;
+		}
+		// also do not proceed if we can't get better than the current shortest
+		// path anymore.
+		if (!shortest.empty() && current->length > shortest.size()) {
+			continue;
+		}
+
+		bool fin = false;
+		if (current->node->isa(&RttiTypes::StructuredClass)) {
+			const StructuredClass *strct =
+			    static_cast<const StructuredClass *>(current->node);
+
+			// look through all fields.
+			NodeVector<FieldDescriptor> fields = strct->getFieldDescriptors();
+			for (auto fd : fields) {
+				// if we found our target, break off the search in this branch.
+				if (fd == target) {
+					fin = true;
+					continue;
+				}
+				// only continue in the TREE field.
+				if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
+					states.push(std::make_shared<PathState>(current, fd.get()));
+				}
+			}
+		} else {
+			// otherwise this is a FieldDescriptor.
+			const FieldDescriptor *field =
+			    static_cast<const FieldDescriptor *>(current->node);
+			// and we proceed by visiting all permitted children.
+			for (auto c : field->getChildrenWithSubclasses()) {
+				// if we found our target, break off the search in this branch.
+				if (c == target) {
+					fin = true;
+					continue;
+				}
+				// We only allow to continue our path via transparent children.
+				if (c->isTransparent()) {
+					states.push(std::make_shared<PathState>(current, c.get()));
+				}
+			}
+		}
+		// check if we are finished.
+		if (fin) {
+			success = true;
+			// if so we look if we found a shorter path than the current minimum
+			if (shortest.empty() || current->length < shortest.size()) {
+				NodeVector<Node> newPath;
+				constructPath(current, newPath);
+				shortest = newPath;
+			} else if (current->length == shortest.size()) {
+				// if the length is the same the result is ambigous and we log
+				// an error.
+				NodeVector<Node> newPath;
+				constructPath(current, newPath);
+				logger.error(
+				    std::string("Can not unambigously create a path from \"") +
+				    start->getName() + "\" to \"" + target->getName() + "\".");
+				logger.note("Dismissed the path:", SourceLocation{},
+				            MessageMode::NO_CONTEXT);
+				for (auto n : newPath) {
+					logger.note(n->getName());
+				}
+			}
+		}
+	}
+	return shortest;
+}
+
+template <typename F>
+static NodeVector<Node> collect(const Node *start, F match)
+{
+	// result
+	NodeVector<Node> res;
+	// queue for breadth-first search of graph.
+	std::queue<Rooted<Node>> q;
+	// put the initial node on the stack.
+	q.push(const_cast<Node *>(start));
+	// set of visited nodes.
+	std::unordered_set<const Node *> visited;
+	while (!q.empty()) {
+		Rooted<Node> n = q.front();
+		q.pop();
+		// do not proceed if this node was already visited.
+		if (!visited.insert(n.get()).second) {
+			continue;
+		}
+
+		if (n->isa(&RttiTypes::StructuredClass)) {
+			Rooted<StructuredClass> strct = n.cast<StructuredClass>();
+
+			// look through all fields.
+			NodeVector<FieldDescriptor> fields = strct->getFieldDescriptors();
+			for (auto fd : fields) {
+				// note matches.
+				if (match(fd)) {
+					res.push_back(fd);
+				}
+				// only continue in the TREE field.
+				if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
+					q.push(fd);
+				}
+			}
+		} else {
+			// otherwise this is a FieldDescriptor.
+			Rooted<FieldDescriptor> field = n.cast<FieldDescriptor>();
+			// and we proceed by visiting all permitted children.
+			for (auto c : field->getChildrenWithSubclasses()) {
+				// note matches.
+				if (match(c)) {
+					res.push_back(c);
+				}
+				// We only continue our search via transparent children.
+				if (c->isTransparent()) {
+					q.push(c);
+				}
+			}
+		}
+	}
+	return res;
+}
+
 /* Class FieldDescriptor */
 
 FieldDescriptor::FieldDescriptor(Manager &mgr, Handle<Type> primitiveType,
@@ -116,7 +311,7 @@ bool FieldDescriptor::doValidate(Logger &logger) const
 	 * there are duplicates.
 	 */
 	std::set<std::string> names;
-	for (Handle<StructuredClass> c : children) {
+	for (Handle<StructuredClass> c : getChildrenWithSubclasses()) {
 		if (!names.insert(c->getName()).second) {
 			logger.error(std::string("Field \"") + getName() +
 			                 "\" had multiple children with the name \"" +
@@ -129,6 +324,25 @@ bool FieldDescriptor::doValidate(Logger &logger) const
 	return valid;
 }
 
+static void gatherSubclasses(NodeVector<StructuredClass> &res,
+                             Handle<StructuredClass> strct)
+{
+	for (auto sub : strct->getSubclasses()) {
+		res.push_back(sub);
+		gatherSubclasses(res, sub);
+	}
+}
+
+NodeVector<StructuredClass> FieldDescriptor::getChildrenWithSubclasses() const
+{
+	NodeVector<StructuredClass> res;
+	for (auto c : children) {
+		res.push_back(c);
+		gatherSubclasses(res, c);
+	}
+	return res;
+}
+
 bool FieldDescriptor::removeChild(Handle<StructuredClass> c)
 {
 	auto it = children.find(c);
@@ -140,6 +354,38 @@ bool FieldDescriptor::removeChild(Handle<StructuredClass> c)
 	return false;
 }
 
+std::pair<NodeVector<Node>, bool> FieldDescriptor::pathTo(
+    Handle<StructuredClass> childDescriptor, Logger &logger) const
+{
+	bool success = false;
+	NodeVector<Node> path =
+	    ousia::pathTo(this, logger, childDescriptor, success);
+	return std::make_pair(path, success);
+}
+NodeVector<Node> FieldDescriptor::pathTo(Handle<FieldDescriptor> field,
+                                         Logger &logger) const
+{
+	bool success = false;
+	return ousia::pathTo(this, logger, field, success);
+}
+NodeVector<FieldDescriptor> FieldDescriptor::getDefaultFields() const
+{
+	// TODO: In principle a cast would be nicer here, but for now we copy.
+	NodeVector<Node> nodes = collect(this, [](Handle<Node> n) {
+		if (!n->isa(&RttiTypes::FieldDescriptor)) {
+			return false;
+		}
+		Handle<FieldDescriptor> f = n.cast<FieldDescriptor>();
+		return f->getFieldType() == FieldDescriptor::FieldType::TREE &&
+		       f->isPrimitive();
+	});
+	NodeVector<FieldDescriptor> res;
+	for (auto n : nodes) {
+		res.push_back(n.cast<FieldDescriptor>());
+	}
+	return res;
+}
+
 /* Class Descriptor */
 
 void Descriptor::doResolve(ResolutionState &state)
@@ -209,152 +455,6 @@ bool Descriptor::doValidate(Logger &logger) const
 	return valid & continueValidationCheckDuplicates(fds, logger);
 }
 
-struct PathState {
-	std::shared_ptr<PathState> pred;
-	Node *node;
-	size_t length;
-
-	PathState(std::shared_ptr<PathState> pred, Node *node)
-	    : pred(pred), node(node)
-	{
-		if (pred == nullptr) {
-			length = 1;
-		} else {
-			length = pred->length + 1;
-		}
-	}
-};
-
-static void constructPath(std::shared_ptr<PathState> state,
-                          NodeVector<Node> &vec)
-{
-	if (state->pred != nullptr) {
-		constructPath(state->pred, vec);
-	}
-	vec.push_back(state->node);
-}
-
-static NodeVector<Node> pathTo(const Descriptor *start, Logger &logger,
-                               Handle<Node> target, bool &success)
-{
-	success = false;
-	// shortest path.
-	NodeVector<Node> shortest;
-	// state queue for breadth-first search.
-	std::queue<std::shared_ptr<PathState>> states;
-	{
-		// initially put every field descriptor on the queue.
-		NodeVector<FieldDescriptor> fields = start->getFieldDescriptors();
-
-		for (auto fd : fields) {
-			if (fd == target) {
-				// if we have found the target directly, return without search.
-				success = true;
-				return shortest;
-			}
-			if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
-				states.push(std::make_shared<PathState>(nullptr, fd.get()));
-			}
-		}
-	}
-	// set of visited nodes.
-	std::unordered_set<const Node *> visited;
-	while (!states.empty()) {
-		std::shared_ptr<PathState> current = states.front();
-		states.pop();
-		// do not proceed if this node was already visited.
-		if (!visited.insert(current->node).second) {
-			continue;
-		}
-		// also do not proceed if we can't get better than the current shortest
-		// path anymore.
-		if (!shortest.empty() && current->length > shortest.size()) {
-			continue;
-		}
-
-		bool fin = false;
-		if (current->node->isa(&RttiTypes::StructuredClass)) {
-			const StructuredClass *strct =
-			    static_cast<const StructuredClass *>(current->node);
-
-			// look through all fields.
-			NodeVector<FieldDescriptor> fields = strct->getFieldDescriptors();
-			for (auto fd : fields) {
-				// if we found our target, break off the search in this branch.
-				if (fd == target) {
-					fin = true;
-					continue;
-				}
-				// only continue in the TREE field.
-				if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
-					states.push(std::make_shared<PathState>(current, fd.get()));
-				}
-			}
-
-			/*
-			 * Furthermore we have to consider that all subclasses of this
-			 * StructuredClass are allowed in place of this StructuredClass as
-			 * well, so we continue the search for them as well.
-			 */
-
-			NodeVector<StructuredClass> subs = strct->getSubclasses();
-			for (auto sub : subs) {
-				// if we found our target, break off the search in this branch.
-				if (sub == target) {
-					fin = true;
-					current = current->pred;
-					continue;
-				}
-				// We only continue our path via transparent classes.
-				if (sub->isTransparent()) {
-					states.push(
-					    std::make_shared<PathState>(current->pred, sub.get()));
-				}
-			}
-		} else {
-			// otherwise this is a FieldDescriptor.
-			const FieldDescriptor *field =
-			    static_cast<const FieldDescriptor *>(current->node);
-			// and we proceed by visiting all permitted children.
-			for (auto c : field->getChildren()) {
-				// if we found our target, break off the search in this branch.
-				if (c == target) {
-					fin = true;
-					continue;
-				}
-				// We only allow to continue our path via transparent children.
-				if (c->isTransparent()) {
-					states.push(std::make_shared<PathState>(current, c.get()));
-				}
-			}
-		}
-		// check if we are finished.
-		if (fin) {
-			success = true;
-			// if so we look if we found a shorter path than the current minimum
-			if (shortest.empty() || current->length < shortest.size()) {
-				NodeVector<Node> newPath;
-				constructPath(current, newPath);
-				shortest = newPath;
-			} else if (current->length == shortest.size()) {
-				// if the length is the same the result is ambigous and we log
-				// an error.
-				NodeVector<Node> newPath;
-				constructPath(current, newPath);
-				logger.error(
-				    std::string("Can not unambigously create a path from \"") +
-				    start->getName() + "\" to \"" + target->getName() + "\".");
-				logger.note("Dismissed the path:", SourceLocation{},
-				            MessageMode::NO_CONTEXT);
-				for (auto n : newPath) {
-					logger.note(n->getName());
-				}
-			}
-		}
-	}
-	return shortest;
-}
-
 NodeVector<Node> Descriptor::pathTo(Handle<StructuredClass> target,
                                     Logger &logger) const
 {
@@ -370,89 +470,6 @@ std::pair<NodeVector<Node>, bool> Descriptor::pathTo(
 	return std::make_pair(path, success);
 }
 
-template <typename F>
-static NodeVector<Node> collect(const Descriptor *start, F match)
-{
-	// result
-	NodeVector<Node> res;
-	// queue for breadth-first search of graph.
-	std::queue<Rooted<Node>> q;
-	{
-		// initially put every field descriptor on the queue.
-		NodeVector<FieldDescriptor> fields = start->getFieldDescriptors();
-
-		for (auto fd : fields) {
-			// note matches.
-			if (match(fd)) {
-				res.push_back(fd);
-			}
-			if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
-				q.push(fd);
-			}
-		}
-	}
-	// set of visited nodes.
-	std::unordered_set<const Node *> visited;
-	while (!q.empty()) {
-		Rooted<Node> n = q.front();
-		q.pop();
-		// do not proceed if this node was already visited.
-		if (!visited.insert(n.get()).second) {
-			continue;
-		}
-
-		if (n->isa(&RttiTypes::StructuredClass)) {
-			Rooted<StructuredClass> strct = n.cast<StructuredClass>();
-
-			// look through all fields.
-			NodeVector<FieldDescriptor> fields = strct->getFieldDescriptors();
-			for (auto fd : fields) {
-				// note matches.
-				if (match(fd)) {
-					res.push_back(fd);
-				}
-				// only continue in the TREE field.
-				if (fd->getFieldType() == FieldDescriptor::FieldType::TREE) {
-					q.push(fd);
-				}
-			}
-
-			/*
-			 * Furthermore we have to consider that all subclasses of this
-			 * StructuredClass are allowed in place of this StructuredClass as
-			 * well, so we continue the search for them as well.
-			 */
-
-			NodeVector<StructuredClass> subs = strct->getSubclasses();
-			for (auto sub : subs) {
-				// note matches.
-				if (match(sub)) {
-					res.push_back(sub);
-				}
-				// We only continue our search via transparent classes.
-				if (sub->isTransparent()) {
-					q.push(sub);
-				}
-			}
-		} else {
-			// otherwise this is a FieldDescriptor.
-			Rooted<FieldDescriptor> field = n.cast<FieldDescriptor>();
-			// and we proceed by visiting all permitted children.
-			for (auto c : field->getChildren()) {
-				// note matches.
-				if (match(c)) {
-					res.push_back(c);
-				}
-				// We only continue our search via transparent children.
-				if (c->isTransparent()) {
-					q.push(c);
-				}
-			}
-		}
-	}
-	return res;
-}
-
 NodeVector<FieldDescriptor> Descriptor::getDefaultFields() const
 {
 	// TODO: In principle a cast would be nicer here, but for now we copy.
@@ -605,7 +622,7 @@ void Descriptor::copyFieldDescriptor(Handle<FieldDescriptor> fd, Logger &logger)
 		copy = Rooted<FieldDescriptor>{
 		    new FieldDescriptor(getManager(), this, fd->getFieldType(),
 		                        fd->getName(), fd->isOptional())};
-		for (auto &c : fd->getChildren()) {
+		for (auto c : fd->getChildren()) {
 			copy->addChild(c);
 		}
 	}
@@ -940,4 +957,4 @@ const Rtti Domain = RttiBuilder<ousia::Domain>("Domain")
                         .parent(&RootNode)
                         .composedOf({&StructuredClass, &AnnotationClass});
 }
-}
+}
\ No newline at end of file
diff --git a/src/core/model/Domain.hpp b/src/core/model/Domain.hpp
index d921a9c..081435a 100644
--- a/src/core/model/Domain.hpp
+++ b/src/core/model/Domain.hpp
@@ -275,6 +275,17 @@ public:
 	const NodeVector<StructuredClass> &getChildren() const { return children; }
 
 	/**
+	 * Returns all StructuredClasses whose instances are allowed as children in
+	 * the Structure Tree of instances of this field including subclasses of
+	 * children, which are allowed directly.
+	 *
+	 * @return all StructuredClasses whose instances are allowed as children in
+	 * the Structure Tree of instances of this field including subclasses of
+	 * children, which are allowed directly.
+	 */
+	NodeVector<StructuredClass> getChildrenWithSubclasses() const;
+
+	/**
 	 * Adds a StructuredClass whose instances shall be allowed as children in
 	 * the StructureTree of instances of this field.
 	 */
@@ -368,6 +379,60 @@ public:
 		invalidate();
 		optional = std::move(o);
 	}
+
+	/**
+	 * This tries to construct the shortest possible path of this Descriptor
+	 * to the given child Descriptor. Note that this method has the problem that
+	 * an empty return path does NOT strictly imply that no such path could
+	 * be constructed: We also return an empty vector if the given
+	 * Descriptor is a direct child. Therefore we also return a bool value
+	 * indicating that the path is valid or not.
+	 *
+	 * Implicitly this does a breadth-first search on the graph of
+	 * StructuredClasses that are transparent. It also takes care of cycles.
+	 *
+	 * @param childDescriptor is a supposedly valid child Descriptor of this
+	 *                        Descriptor.
+	 * @return                a tuple containing a path of FieldDescriptors and
+	 *                        StructuredClasses between this Descriptor and the
+	 *                        input Descriptor and a bool value indicating if
+	 *                        the construction was successful.
+	 *
+	 */
+	std::pair<NodeVector<Node>, bool> pathTo(
+	    Handle<StructuredClass> childDescriptor, Logger &logger) const;
+	/**
+	 * This tries to construct the shortest possible path of this Descriptor
+	 * to the given FieldDescriptor. Note that this method has the problem that
+	 * an empty return path does NOT strictly imply that no such path could
+	 * be constructed: We also return an empty vector if the given
+	 * FieldDescriptor is a direct child. Therefore we also return a bool value
+	 * indicating that the path is valid or not.
+	 *
+	 *
+	 * Implicitly this does a breadth-first search on the graph of
+	 * StructuredClasses that are transparent. It also takes care of cycles.
+	 *
+	 * @param field is a FieldDescriptor that may be allowed as child of this
+	 *              Descriptor.
+	 * @return      a path of FieldDescriptors and StructuredClasses between
+	 *              this Descriptor and the input FieldDescriptor or an empty
+	 *              vector if no such path could be constructed.
+	 */
+	NodeVector<Node> pathTo(Handle<FieldDescriptor> field,
+	                        Logger &logger) const;
+
+	/**
+	 * Returns a vector of all TREE fields that are allowed as structure tree
+	 * children of an instance of this Descriptor. This also makes use of
+	 * transparency.
+	 * The list is sorted by the number of transparent elements that have to be
+	 * constructed to arrive at the respective FieldDescriptor.
+	 *
+	 * @return a vector of all TREE fields that are allowed as structure tree
+	 *         children of an instance of this Descriptor.
+	 */
+	NodeVector<FieldDescriptor> getDefaultFields() const;
 };
 
 /**
@@ -1111,5 +1176,4 @@ extern const Rtti Domain;
 }
 }
 
-#endif /* _OUSIA_MODEL_DOMAIN_HPP_ */
-
+#endif /* _OUSIA_MODEL_DOMAIN_HPP_ */
\ No newline at end of file
diff --git a/src/core/model/Typesystem.cpp b/src/core/model/Typesystem.cpp
index fb99f87..df2b9fb 100644
--- a/src/core/model/Typesystem.cpp
+++ b/src/core/model/Typesystem.cpp
@@ -109,6 +109,14 @@ bool StringType::doBuild(Variant &data, Logger &logger,
 	return VariantConverter::toString(data, logger);
 }
 
+/* Class CardinalityType */
+
+bool CardinalityType::doBuild(Variant &data, Logger &logger,
+                              const MagicCallback &magicCallback) const
+{
+	return VariantConverter::toCardinality(data, logger);
+}
+
 /* Class EnumType */
 
 EnumType::EnumType(Manager &mgr, std::string name, Handle<Typesystem> system)
@@ -769,12 +777,14 @@ SystemTypesystem::SystemTypesystem(Manager &mgr)
       stringType(new StringType(mgr, this)),
       intType(new IntType(mgr, this)),
       doubleType(new DoubleType(mgr, this)),
-      boolType(new BoolType(mgr, this))
+      boolType(new BoolType(mgr, this)),
+      cardinalityType(new CardinalityType(mgr, this))
 {
 	addType(stringType);
 	addType(intType);
 	addType(doubleType);
 	addType(boolType);
+	addType(cardinalityType);
 }
 
 /* RTTI type registrations */
@@ -787,6 +797,8 @@ const Rtti IntType = RttiBuilder<ousia::IntType>("IntType").parent(&Type);
 const Rtti DoubleType =
     RttiBuilder<ousia::DoubleType>("DoubleType").parent(&Type);
 const Rtti BoolType = RttiBuilder<ousia::BoolType>("BoolType").parent(&Type);
+const Rtti CardinalityType =
+    RttiBuilder<ousia::CardinalityType>("CardinalityType").parent(&Type);
 const Rtti EnumType = RttiBuilder<ousia::EnumType>("EnumType").parent(&Type);
 const Rtti StructType = RttiBuilder<ousia::StructType>("StructType")
                             .parent(&Type)
@@ -798,10 +810,9 @@ const Rtti Constant = RttiBuilder<ousia::Constant>("Constant").parent(&Node);
 const Rtti Attribute = RttiBuilder<ousia::Attribute>("Attribute").parent(&Node);
 const Rtti Typesystem =
     RttiBuilder<ousia::Typesystem>("Typesystem").parent(&RootNode).composedOf(
-        {&StringType, &IntType, &DoubleType, &BoolType, &EnumType, &StructType,
-         &Constant});
+        {&StringType, &IntType, &DoubleType, &BoolType, &CardinalityType,
+         &EnumType, &StructType, &Constant});
 const Rtti SystemTypesystem = RttiBuilder<ousia::SystemTypesystem>(
                                   "SystemTypesystem").parent(&Typesystem);
 }
-}
-
+}
\ No newline at end of file
diff --git a/src/core/model/Typesystem.hpp b/src/core/model/Typesystem.hpp
index 53fb0df..39f777f 100644
--- a/src/core/model/Typesystem.hpp
+++ b/src/core/model/Typesystem.hpp
@@ -386,6 +386,55 @@ public:
 };
 
 /**
+ * The CardinalityType class represents the cardinality type. There should be
+ * exactly one instance of this class available in a preloaded type system.
+ */
+class CardinalityType : public Type {
+protected:
+	/**
+	 * Expects the given variant to be a cardinality or a single int.
+	 *
+	 * @param data is a variant containing the data that should be checked.
+	 * @param logger is the Logger instance into which errors should be written.
+	 * @return true if the conversion was successful, false otherwise.
+	 */
+	bool doBuild(Variant &data, Logger &logger,
+	             const MagicCallback &magicCallback) const override;
+
+public:
+	/**
+	 * Constructor of the CardinalityType class. Only one instance of
+	 *CardinalityType should
+	 * exist per project graph.
+	 *
+	 * @param mgr is the Manager instance to be used for the Node.
+	 * @param name is the name of the type.
+	 * @param system is a reference to the parent Typesystem instance.
+	 */
+	CardinalityType(Manager &mgr, Handle<Typesystem> system)
+	    : Type(mgr, "cardinality", system, true)
+	{
+	}
+
+	/**
+	 * Creates a variant with the cardinality value "any".
+	 *
+	 * @return a Variant with the cardinality value "any".
+	 */
+	Variant create() const override { return Variant{Cardinality::any()}; }
+
+	/**
+	 * Returns the cardinality VariantType.
+	 *
+	 * @return the cardinality VariantType.
+	 */
+	std::vector<VariantType> getVariantTypes() const override
+	{
+		return {VariantType::CARDINALITY};
+	}
+};
+
+/**
  * The EnumType class represents a user defined enumeration type.
  */
 class EnumType : public Type {
@@ -1298,6 +1347,11 @@ private:
 	 */
 	Handle<BoolType> boolType;
 
+	/**
+	 * Reference to the cardinality type.
+	 */
+	Handle<CardinalityType> cardinalityType;
+
 public:
 	/**
 	 * Creates the SystemTypesystem containing all basic types (string, int,
@@ -1335,6 +1389,13 @@ public:
 	 * @return a reference to the primitive BoolType instance.
 	 */
 	Rooted<BoolType> getBoolType() { return boolType; }
+
+	/**
+	 * Returns the cardinality type.
+	 *
+	 * @return a reference to the CardinalityType instance.
+	 */
+	Rooted<CardinalityType> getCardinalityType() { return cardinalityType; }
 };
 
 /* RTTI type registrations */
@@ -1366,6 +1427,11 @@ extern const Rtti DoubleType;
 extern const Rtti BoolType;
 
 /**
+ * Type information for the CardinalityType class.
+ */
+extern const Rtti CardinalityType;
+
+/**
  * Type information for the EnumType class.
  */
 extern const Rtti EnumType;
@@ -1407,5 +1473,4 @@ extern const Rtti SystemTypesystem;
 }
 }
 
-#endif /* _OUSIA_MODEL_TYPESYSTEM_HPP_ */
-
+#endif /* _OUSIA_MODEL_TYPESYSTEM_HPP_ */
\ No newline at end of file