If I have a an N^3 array of triplets in a numpy array, how do I do a vector sum on all of the triplets in the array? For some reason I just can't wrap my brain around the summation indices. Here is what I tried, but it doesn't seem to work:

```
a = np.random.random((5,5,5,3)) - 0.5
s = a.sum((0,1,2))
np.linalg.norm(s)

```

I would expect that as N gets large, if the sum is working correctly I should converge to 0, but I just keep getting bigger. The sum gives me a vector that is the correct shape (3x1), but obviously I must be doing something wrong. I know this should be easy, but I'm just not getting it.

Thanks in advance!