
  [;1m-spec family_to_digraph(Family) -> Graph[0m
  [;1m                           when[0m
  [;1m                               Graph :: digraph:graph(),[0m
  [;1m                               Family :: family().[0m

  Creates a directed graph from family [;;4mFamily[0m. For each pair
  (a, {b[1], ..., b[n]}) of [;;4mFamily[0m, 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, [;;4mdigraph:new/0[0m is used for
  creating the directed graph, otherwise argument [;;4mGraphType[0m is
  passed on as second argument to [;;4mdigraph:new/1[0m.

  It F is a family, it holds that F is a subset of [;;4m[0m
  [;;4mdigraph_to_family(family_to_digraph(F), type(F))[0m. Equality holds
  if [;;4munion_of_family(F)[0m is a subset of [;;4mdomain(F)[0m.

  Creating a cycle in an acyclic graph exits the process with a [;;4m[0m
  [;;4mcyclic[0m message.
