{
Map<ElementGraph, EnumMultiMap> annotatedSubGraphs = new LinkedHashMap<>();
// need a safe copy
if( elementGraph.containsVertex( Extent.head ) )
elementGraph = new ElementMaskSubGraph( elementGraph, Extent.head, Extent.tail );
annotatedSubGraphs.put( new ElementDirectedGraph( elementGraph ), new EnumMultiMap() );
return new Partitions( this, elementGraph, annotatedSubGraphs );
}