/// <summary>
/// Removes outdated key events from a changeset, only leaving the last relevent change for each key.
/// </summary>
public static IChangeSet <TObject, TKey> EnsureUniqueChanges <TObject, TKey>(this IChangeSet <TObject, TKey> input)
{
var changes = input
.GroupBy(kvp => kvp.Key)
.Select(g => g.Aggregate(Optional <Change <TObject, TKey> > .None, Reduce))
.Where(x => x.HasValue)
.Select(x => x.Value);
return(new ChangeSet <TObject, TKey>(changes));
}