prolog need to compute the tree size -

i need size of tree using: size(tree,size)

what have far wrong, please advise!

size(empty, size). size(tree(l, _, r), size) :-     size(l, left_size),     size(r, right_size),     size         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.

prolog declarative language, must state correctly patterns:

size(node(l,r), size) :- ... % why add 1 left+right sizes ?

from samples, suggest stop recursion size = 1 when not node seen:

size(node(l,r), size) :- !, ... size(_, 1).

Script