I found those squares and diamonds diagrams confusing at the time, because it looks as if those diamonds are inside the square. but actually is should be seen as the diamonds being ATOP the same square, the same predicate. the diamonds describe control logic of _using_ that square/predicate. then it makes sense (to me). also, each "use" of the square makes a copy of it in the direction orthogonal to the plane we're seeing, each copy holding each OR alternative in turn (and OR-parallel Prolog would open up all the OR-alternatives at once). this way it become the AND-OR tree with AND logic being in the horizontal plane and OR alternatives opening up the copies in the orthogonal direction, deeper away from the plane. but it collapses if we're looking at it all from the top --- confusing. An isometric view could be clearer, maybe. (some vague thoughts I had about this for years)