/// <summary>
/// Merge data object with this list.
/// Only objects that are not already in the list
/// are added to the list.
/// </summary>
/// <param name='data'>The data to merge.</param>
public void Merge(WebFactorTreeNode data)
{
if (data.IsNotNull() && _idHashTable[data.Id].IsNull())
{
Add(data);
}
}
