how is the grass done for super mario world?

i noticed the grass in mario seems to have an alpha grass running along any border of the tiling grass texture in the game but its seamless.

my thoughts on how this was done is by having a stip of polygons around the border edges of the grass, and that stip has alpha on both sides? one side shows the tips of the grass and the other side i'm (guessing) has the poly edge slight raised above the tiling grass texture and with the alpha also on that side it fades off?

how the hell do they do this? maybe this is just some old school technique? i'd love to do grass similar to this for an environment i'm working on. love the grass in this game.