public Arc AddArc(Node from, Node to, IGraphTag tag = null)
{
if (from.GetArcTo(to) != null)
{
return(null);
}
var newArc = new Arc(from, to, tag);
from.ArcsOut.Add(newArc);
to.ArcsIn.Add(newArc);
Arcs.Add(newArc);
return(newArc);
}