Edge :
======

   Edges are directed broken lines (not splines). They join any couple
of objects with types in {port, web, vertex}. They bear possibly
several labels (implicitly representing multi-edges).

   To create an edge:
1. select the <ObjectsEdit>:<Edge>:<Create (e)> menu command 
     ('e' is the keyboard shortcut);
2. click inside the source object (with left mouse button), keep button down;
3. Each mouse button release will create a new pinning nail, except when
   a proper target object, where the edition will end.

   Nails may be added, moved or killed by menu.

   The menu commands <ObjectsEdit>:<Edge>:<change Source> and
<ObjectsEdit>:<Edge>:<change Target> will allow to redirect a given
edge.  CAUTION: not yet implemented.

   The menu commands <ObjectsEdit>:<Edge>:<Show Source> and
<ObjectsEdit>:<Edge>:<Show Target> highlight the corresponding
objects.
