//returns true if tree is valid after remove
public bool removeNeighbour(Node node)
{
node.neighbours.Remove(this);
neighbours.Remove(node);
return isValid() && node.isValid();
}
//returns true if tree is valid after add
public bool addNeighbour(Node node)
{
neighbours.AddLast(node);
node.neighbours.AddLast(this);
return isValid() && node.isValid();
}
