Rewrite recursive sicstus prolog function

Question

My goal is to have this input:

L = [a,b,c], build_tree(L,T).

With this output:

L = [1,30,kth,5],
T = b(l(a),b(l(b),b(l(c)))) ? 

yes

And with this code, that counts the number of leaves in a tree:

leaves(l(X), [X]).
leaves(b(L1,L2),V):-
    leaves(L1,V1),
    leaves(L2,V2),
    append(V1,V2,V).

I can get the desired output by simply giving the function a list instead of a tree as input, eg:

L = [a,b,c], leaves(T,L).

The only problem here is that it takes the arguments in the wrong order (i.e. build_tree(T,L) instead of build_tree(L,T)).

So, how can I produce the same result, but simply swap the input arguments? I've tried every "obvious" solution (swapping around variables), but I'm guessing that it might not be as easy as it seems since it's a recursive method.