/// <summary>
/// Performs a "union" of two sets, where all the elements
/// in both are present. That is, the element is included if it is in either <c>a</c> or <c>b</c>.
/// The return value is a <c>Clone()</c> of one of the sets (<c>a</c> if it is not <c>null</c>) with elements of the other set
/// added in. Neither of the input sets is modified by the operation.
/// </summary>
/// <param name="a">A set of elements.</param>
/// <param name="b">A set of elements.</param>
/// <returns>A set containing the union of the input sets. <c>null</c> if both sets are <c>null</c>.</returns>
public static Set Union(Set a, Set b)
{
if (a == null && b == null)
{
return(null);
}
else if (a == null)
{
return((Set)b.Clone());
}
else if (b == null)
{
return((Set)a.Clone());
}
else
{
return(a.Union(b));
}
}
