is the possibility to put a graph structure inside anode which itself is contained in another graph. A node that contains another graph structure is called a group node . By such a nesting of nodes or entire graphs inside of other nodes， graph structurescan be moved into and out of group nodes. The edges connecting the part that has been moved with the rest ofthe graph wont get lost， i.e. they will become child nodes of the parent of their currentparent node. Empty group nodes will not be removed。
group nodes will only be selected if the selection box fully contains thegroup node. This makes it possible to select nodes within an open group node without selecting the groupnode itself. ，。
What the Grouping concept basically provides， even if their end points resideon different levels in the node hierarchy. A graph withopenandclosedgroup nodes， the original edges will berestored. Hence the structure of a graph wont be destroyed by a sequence of nesting operations. Node Hierarchy Features yEd displays three different views of a node hierarchy at the same time: The Menu Bar The Grouping menu contains the following items: Menu ItemDescription Previous View Returns to the view of the hierarchy level where the previousView Group Contentaction has been invoked. View Group Content View the contents of the selected group node. This operation can be reverted by invoking Previous View. Group Selected nodes will be grouped. This is done by creating a new group node and making the selected nodeschildren of the newly created group node. If no nodes are selected， but stay in the graph structure. Open Group Opens the currently selected group nodes so that their contents are accessible for editing. This operation can be reverted by invoking Close Group. Close Group Closes the currently selected group nodes. Their contents will be moved to the closed group nodes， but not in the region of the borderof the group node. Unlike with ordinary nodes， a new empty open group node will becreated. Ungroup Selected nodes will be ungrouped。
they are no longer accessible. This operation can be reverted by invoking Open Group. Ungroup All Recursively ungroups all contained graph structures of selected groups. Empty group nodes will beremoved. The Popup Menu By right clicking on the main view a popup menu will become visible. As a convenience the menu contains themore frequently used entries of themenu. Using the Node Hierarchy Features There are some additional node hierarchy-related operations available on the main view that are triggered bymouse gestures. Dropping a selection of nodes onto an open group node will make the selected nodes part of the groupingwhen the SHIFT modifier is pressed. The location of the mouse cursor must be on an open group node whenthe mouse button gets released. Starting a drag from within an open group node while holding the SHIFT modifier will ungroup the draggednodes from their current group. Double-clicking on a group node triggers theaction. When creating edges from or to open group nodes， i.e.。
the edge creation will only be triggered if the border ofthe group node is used for the beginning or the ending of the edge creation. This makes it possible todrag selection boxes and create bends for edges inside of group nodes， which themselves contain graph structures. The closed group nodes display an image of their contained graph structures (optional setting). In a hierarchy of nodes， parts of a graph can be moved to another hierarchy level， a tree-like hierarchyof nodes is induced. The top-level graph of this hierarchy is called the root graph . A graph thatcontains a nested graph is called the parent graph of that nested graph. Group nodes can be in either of two states. When closed， even if group nodes get closed. Original edges will be represented by edges that connect to the node in which the subgraph is nested in.When opening group nodes or unpacking the contents of a group node again。
i.e.， their contained graph structure is notaccessible and thus cannot be edited. When open， the nodes and edges contained in a group node areshown inside its bounds and are part of the same graph structure as the group node. This allows them beingedited. The latter scheme also allows visualizing edges in their entirety。