Pr. Euler 18, recursion problem

[process]

I am trying to solve project euler problem 18 with brute force(I will
move on to a better solution after I have done that for problem 67).
http://projecteuler.net/index.php?section=problems&id=18

However I can't get the recursive function right.

I always have to use return right? Unless I am printing? So I canät
stack to diffferent recursive calls after each other like so:
recur_left(t, p)
recur_right(t,p+1)


Some stuff I have tried:

def recur(tree, pos):
    if not tree:
        return []
    else:
        return [[tree[0][pos]] + recur(tree[1:], pos)] + \
               [[tree[0][pos]] + recur(tree[1:], pos+1)]


>>> recur([[1],[2,3],[4,5,6]],0)
[[1, [2, [4], [4]], [2, [5], [5]]], [1, [3, [5], [5]], [3, [6], [6]]]]


SO it is kind of working, just not exactly like I want.
A more easily parseable/readable result would be nice, I want to be
able to sum() over each path preferrably.

So the result should be:
[[1,2,4],[1,2,5],[1,3,5],[1,3,6]]



I know conceptually what has to be done.
Base case: empty tree, return []
Else: recur to the left and to the right.