/// <summary>
 /// Method for collecting all distinct system (orbital) indices.
 /// </summary>
 /// <param name="orbitalIntegral">Collate orbital indices from this orbital integral.</param>
 public override void AddToSystemIndices(HermitianFermionTerm index)
 {
     foreach (var idx in index.Sequence)
     {
         SystemIndices.Add(idx.Index);
     }
 }
 /// <summary>
 /// Method for collecting all distinct system (orbital) indices.
 /// </summary>
 /// <param name="orbitalIntegral">Collate orbital indices from this orbital integral.</param>
 public override void AddToSystemIndices(OrbitalIntegral index)
 {
     foreach (var idx in index.OrbitalIndices)
     {
         SystemIndices.Add(idx);
     }
 }