 For each such relation, we may also define a computable tree \(\textit{Tr}_x\) consisting of the finite sequences \(\sigma \in \mathbb{N}^{&lt; \mathbb{N}}\) such that for all proper initial subsequences \(\tau \subset \sigma\), \(\neg R(x,\tau)\) holds. A leaf node in this tree thus corresponds to the first place for which \(R\) holds. An infinite path in \(\textit{Tr}_x\) thus corresponds to a function \(f\) such that \(\forall y \neg R(x,\overline{f}(y))\), which is in turn a witness to \(x \not\in A\). It thus follows that \(x \in A\) if and only if \(\textit{Tr}_x\) is well-founded. Since it is straightforward to effectively enumerate computable trees, it is also not difficult to show the following: