Specifying naturally recursive definitions without switching to iterative

Right now in the DSL recursive definitions are disallowed. This mostly makes sense: we don't support general recursion due to everything being fixed space, and we don't have any tail call detection / optimization facilities. However, there are a bunch of algorithms that lend themselves nicely to a recursive definition that we should find a way to express so we have have to _force_ everything to be expressed iteratively; i.e. leading zeros detection with a balanced tree could be a good one: https://electronics.stackexchange.com/a/412278

No file, test, or entry point is named. Start by locating where the DSL rejects recursive definitions and documenting the fixed-space and tail-call constraints; the work is complete only when a concrete, supported form of natural recursion and its acceptance criteria are defined and tested.

Written by the indexing model from the issue text.