To create a new arc:
An arc of the specified type is created from the starting node to the ending node.
The current selection set is not disturbed.
Tip: To create a self arc, when the starting and ending nodes are the same, drag the pointer to the top of the node and release.
Trying to ``create'' an already existing arc merely selects the arc.
Note: You cannot directly create level arcs in any window; level arcs are only created when you collapse nodes into a subsystem.
Only up to one arc is permitted between a given source and destination; multi-arcs are not supported.