A B+ tree as a rotary card file. The root and interior trays hold only guide cards; every row lives in a leaf, and the leaves hang side by side on the rim, chained by next. A lookup reads one tray per level. A range scan just turns the wheel along the chain, and one that runs past the last leaf follows next = NULL to an empty hanger: segmentation fault.

for (leaf = find(lo); leaf->keys[0] <= hi; leaf = leaf->next)   /* no NULL check */