/// <summary>
/// <para>Return list, reversed or not, with elements not fulfilling [f] removed.
/// Avoid allocation if possible.
///
/// </para>
/// </summary>
public list <T> RevFilterWhenNeeded(Function <T, bool> f)
{
return((!NList.ForAll <T>(this, f)) ? NList.RevFilter <T>(this, f) : this);
}
/// <summary>
/// <para>Removes elements for which predicate is false.
/// The resulting list is reversed (operation is faster this way).
///
/// </para>
/// </summary>
public list <T> RevFilter(Function <T, bool> f)
{
return(NList.RevFilter <T>(this, f));
}