I need to get the size of a tree using: `size(Tree,Size)`

What I have so far is wrong, please advise!

```
size(empty, Size).
size(tree(L, _, R), Size) :-
size(L, Left_Size),
size(R, Right_Size),
Size is
Left_Size + Right_Size + 1.
```

Output should produce:

```
?- size(node(1,2),X).
X = 2.
?- size(node(1,[2,3,4]),X).
X = 2.
?- size(node(node(a,b),[2,3,4]),X).
X = 3.
```