code: public Collection<GroupTransition> getTransitives(String groupIn, String groupOut)
      {
          final GroupTransition transition = new GroupTransition(groupIn, groupOut);
          if (!transitives.containsKey(transition))
          {
              return Collections.emptyList();
          }
          return transitives.get(transition);
      }
docs: Get the transitive groups list to reach a group from another.
  
  @param groupIn The first group.
  @param groupOut The last group.
  @return The transitive groups.