WIP: Broken arc implementation.

smtk/markup/CMakeLists.txt

 set(sources)
 set(headers
+  arcs/Labels.h
+
   queries/SelectionFootprint.h
 )
 set(operations

smtk/markup/Traits.h

@@ -47,6 +47,7 @@
 #include "smtk/markup/Subset.h"
 #include "smtk/markup/SurfaceGrid.h"
 #include "smtk/markup/VolumeGrid.h"
+#include "smtk/markup/arcs/Labels.h"
 #include "smtk/markup/detail/NodeContainer.h"
 
 #include <tuple>
@@ -98,6 +99,8 @@ struct SMTKMARKUP_EXPORT Traits
     VolumeGrid
   >;
   using ArcTypes = std::tuple<
+    Labels,
+    LabelSubject
   >;
   /*
   typedef std::tuple<ChildrenAs<CompSolid>, ChildrenAs<Solid>, ChildrenAs<Shell>, ChildrenAs<Face>,

smtk/markup/arcs/Labels.h

+// Copyright © Kitware Inc under the [BSD-3-clause license](https://opensource.org/licenses/BSD-3-Clause).
+#ifndef smtk_markup_arcs_Labels_h
+#define smtk_markup_arcs_Labels_h
+
+#include "smtk/markup/Exports.h"
+#include "smtk/graph/arcs/Arc.h"
+#include "smtk/graph/arcs/Arcs.h"
+
+namespace smtk
+{
+namespace markup
+{
+
+// Node types
+class Component;
+class Label;
+
+// Forward declare arc types
+class Labels;
+class LabelSubject;
+
+class SMTKMARKUP_EXPORT Labels : public smtk::graph::Arcs<Component, Label, Labels>
+{
+public:
+  using InverseArcType = LabelSubject;
+  using smtk::graph::Arcs<Component, Label, Labels>::Arcs;
+};
+
+class SMTKMARKUP_EXPORT LabelSubject : public smtk::graph::Arc<Label, Component, LabelSubject>
+{
+public:
+  using InverseArcType = Labels;
+  using smtk::graph::Arc<Label, Component, LabelSubject>::Arc;
+};
+
+} // namespace markup
+} // namespace smtk
+
+#endif // smtk_markup_arcs_Labels_h

smtk/markup/testing/cxx/TestResource.cxx

@@ -38,6 +38,9 @@ int TestResource(int argc, char** argv)
   nn = components.size();
   std::cout << "Found " << nn << " labels\n";
 
+  // resource->connect<LabelSubject>(*label, *group);
+  label->get<LabelSubject>().insert(*group);
+
   auto write = smtk::markup::Write::create();
   write->parameters()->associations()->appendValue(resource);
   write->operate();