
  -spec digraph_to_family(Graph, Type) -> Family
                             when
                                 Graph :: digraph:graph(),
                                 Family :: family(),
                                 Type :: type().

  Creates a family from the directed graph Graph. Each vertex a
  of Graph is represented by a pair (a, {b[1], ..., b[n]}), where
  the b[i]:s are the out-neighbors of a. If no type is explicitly
  specified, [{atom, [atom]}] is used as type of the family. It is
  assumed that Type is a valid type of the external set of the
  family.

  If G is a directed graph, it holds that the vertices and edges of
  G are the same as the vertices and edges of 
  family_to_digraph(digraph_to_family(G)).
