It's not hard to see how it works. First thing to realize that one way of calculating the next line in the triangle is to take the previous line twice, shift one of the line one position to the right, and then add the elements piecewise. For instance:

    1  3  3  1
       1  3  3  1
    ------------- +
    1  4  6  4  1

But if you look carefully, you see that to each element, we add the element to the left of it, except for the two elements on the far ends - which will both be one (a "new" 1 on the right, and the one of the left remains "as is").

And that's how the program works. We first add a new element (with the push), then for each element, we add the preceding one. We have to work backwards of course, which is achieved by the reverse.

Abigail