/// <summary>
/// an expandwalk with roots of thing
/// attention! use it only once, or make tolist or toarray
/// </summary>
/// <param name="thing"></param>
/// <returns></returns>
public IEnumerable <IThing> Roots(IThing thing)
{
return(ThingGraph.Walk()
.ExpandWalk(thing, 0, Walk.Roots <IThing, ILink> ())
.Where(item => !(item.Node is ILink || item.Node == thing))
.Select(item => item.Node));
}