
  -spec family_to_digraph(Family) -> Graph
                             when
                                 Graph :: digraph:graph(),
                                 Family :: family().

  Creates a directed graph from family Family. For each pair
  (a, {b[1], ..., b[n]}) of Family, vertex a and the edges
  (a, b[i]) for 1 <= i <= n are added to a newly created directed
  graph.

  If no graph type is specified, digraph:new/0 is used for
  creating the directed graph, otherwise argument GraphType is
  passed on as second argument to digraph:new/1.

  It F is a family, it holds that F is a subset of 
  digraph_to_family(family_to_digraph(F), type(F)). Equality holds
  if union_of_family(F) is a subset of domain(F).

  Creating a cycle in an acyclic graph exits the process with a 
  cyclic message.
