public int?Compare(INodeConstraint other)
        {
            if (other == null || other == Unconstrained.Instance)
            {
                return(-1);
            }

            return(null);
        }
 /// <summary>
 /// Adds a constraint for nodes.
 /// </summary>
 /// <param name="constraint"></param>
 public void AddNodeConstraint(INodeConstraint <TLayout, TNode, TConfiguration, TEnergyData> constraint)
 {
     nodeConstraints.Add(constraint);
 }