Edges are broken arrows, which may join any 
couple of objects with types in {port,web,vertex}.

  How to create an edge:
1- select the "edition:edge:create" menu function.
2- designates the source object with left mouse
   button (keeping the button down). Each successive
   button release will then add a new angle point
   to the broken line, until the button is finally
   released inside an appropriate target object.

  Points may be added, moved or killed with proper
menu functions.

  Edges are selected/moved/killed alongside with
their endpoint objects. When only one endpoint is
selected, the edge is reshaped from the first point
encountered.

  Unlike other objects edges may be multiply
labelled, saving line occupancy in the drawing area.

  An edge has default colors:
      Atg*edgeforeground
Further edge attributes may alter color shades.

 The shape of the arrow head also depends on X11
resources, for length:
  Atg*arrowradius
and angle:
  Atg*arrowdelta

 Edge are unidimensional objects, and therefore 
hard to be accurately clicked on. A resource value
  Atg*deltaline
allow to seize an edge "up to" so many pixels.

 Relevance of edges (for instance joining a
vertex to a port) is NOT checked until explicit
translation/completion is required by the user.

 When system:flags:align is on, point locations
are straightened at move to align as much as 
possible respective to outgoing edge segments.

WARNING: not yet implemented:
The menu functions Edition:edge:changeSource
and Edition:edge:changeTarget allow to redirect
a given edge from the point where it gets selected.
The menu functions Edition:edge:showSource
and Edition:edge:showTarget simply highlight the
corresponding objects.