add(null,0):-!.
add(tree(X,T1,T2),S):- add(T1,S1),add(T2,S2),S is X+S1+S2.

count_leaves(null,0):-!.
count_leaves(tree(_,null,null),1):-!.
count_leaves(tree(_,Left,Right),Count):-count_leaves(Left,Count1),
    count_leaves(Right,Count2),Count is Count1 + Count2.

compute(tree(V,null,null),V).
compute(tree(+,Left,Right),V):-compute(Left,R1),compute(Right,R2),V is R1+R2.
compute(tree(-,Left,Right),V):-compute(Left,R1),compute(Right,R2),V is R1-R2.
compute(tree(*,Left,Right),V):-compute(Left,R1),compute(Right,R2),V is R1*R2.
compute(tree(/,Left,Right),V):-compute(Left,R1),compute(Right,R2),V is R1/R2.

converting(V, tree(V,null,null)):-number(V).
converting(X-Y,tree(-,Left,Right)):-converting(X,Left),converting(Y,Right).
converting(X+Y,tree(+,Left,Right)):-converting(X,Left),converting(Y,Right).
converting(X*Y,tree(*,Left,Right)):-converting(X,Left),converting(Y,Right).
converting(X/Y,tree(/,Left,Right)):-converting(X,Left),converting(Y,Right).