Others have described simple recursive data structures, but a real-world example is in the MTOSI definition of network modeling. Equipment Holders (EH), eg slot

Others have described simple recursive data structures, but a real-world example is in the MTOSI definition of network modeling. Equipment Holders (EH), eg slots, can hold Equipment (EQ), eg cards. EHs can also hold other EHs arbitrarily deep, and EQ can hold EQ as well as EHs. So they are mutually recursive. A standard way to walk such data structures is with mutually recursive functions. In other words, a natural way to talk about recursive data structures is with recursive functions and this is made more obvious with data structures that are mutually recursive.