The polymorphic recursion allows for compile-time checking of the invariant that "the left and right wings at depth n are 2-3 trees of depth n". In C++, I think

The polymorphic recursion allows for compile-time checking of the invariant that "the left and right wings at depth n are 2-3 trees of depth n". In C++, I think you would forgo a compile-time check of that invariant and just write code that maintains it instead.