[eadmund]

> OOP will let you express this kind of problem from a data modeling perspective (List<T> on each type), but from a serialization and dependency perspective you have to pick a "winner". In banking, it is unclear which type should be king.

Serialisation is not necessarily a big deal, just pick one and refer back to it — check out PRINT-CIRCLE, Sharpsign Equal-Sign and Sharpsign Sharpsign in Common Lisp.

For dependencies, I think this only matters with strong type systems which don’t support forward declarations, or which lack null references/empty containers (if the language does support that, just create one object without any references, create the second, then add the second to the first).