/// <summary>
/// Retrieves the minimum key and the associated value.
/// </summary>
/// <returns>The tuple of the minimum key, associated value and the
/// dictionary containing all other elements, if the dictionary is
/// non-empty; <c>None</c> otherwise.</returns>
public Optional <Tuple <TKey, TValue> > MinKeyAndValue()
{
if (Count == 0)
{
return(Optional <Tuple <TKey, TValue> > .None);
}
if (Count == 1)
{
return(Optional.Some(Tuple.New(_key, _value)));
}
return(_left.MinKeyAndValue());
}