void RemoveReference(IPersonReference reference)
        {
            References.Remove(reference);
            PatternWords.Remove(reference);
            NominativePersons.Remove(reference);

            ReferencesSelectedItem = null;
            UpdateLookup(null);
        }
 void UpdatePersonHeaders()
 {
     ErrorsVisibility      = PatternWords.Any() ? Visibility.Visible : Visibility.Collapsed;
     NominativesVisibility = NominativePersons.Any() ? Visibility.Visible : Visibility.Collapsed;
 }