Torus

Loops can be considered as sets or spaces more closely than other kinds of lists because every item is geometrically indistinguishable - none are the start or end; there's no center, inside or outside. In geometric terms, all the points of the loop form the surface of a 1-sphere.

The loops actually form a hierarchy, since each item in a loop can also be a loop. Geometrically this forms a recursive torus - a torus is a circle where all the points composing the circumference are also circles.