/// <summary> Takes in a form index which is a subset of this index, and returns the
/// total difference between them. This is useful for stepping up the level
/// of index specificty. If the subIndex is not a valid subIndex of this index,
/// null is returned. Since the FormIndex represented by null is always a subset,
/// if null is passed in as a subIndex, the full index is returned
///
/// For example:
/// Indices
/// a = 1_0,2,1,3
/// b = 1,3
///
/// a.diff(b) = 1_0,2
///
/// </summary>
/// <param name="subIndex">
/// </param>
/// <returns>
/// </returns>
public virtual FormIndex diff(FormIndex subIndex)
{
if (subIndex == null)
{
return(this);
}
if (!isSubIndex(this, subIndex))
{
return(null);
}
if (subIndex.Equals(this))
{
return(null);
}
return(new FormIndex(nextLevel.diff(subIndex), this.snip()));
}
