public PersistentMap <K, T> Assign(K key, T value)
{
if (root == null)
{
return(new PersistentTreapMap <K, T>(new TreapNode(key, value)));
}
TreapNode y = root.Find(key);
if (y == null)
{
return(new PersistentTreapMap <K, T>(root.Insert(new TreapNode(key, value))));
}
return(new PersistentTreapMap <K, T>(root.Change(key, value)));
}