A PCAC puzzle: $\mathrm{\pi ⁰\; \to \; \gamma \gamma }$ in the $\sigma$-model

Bell and Jackiw’s A PCAC puzzle: π⁰ → γγ in the σ-model (1969) is part of the program’s reference corpus, acknowledged as a formal antecedent in Foundations and Logic whose structures inform the framework’s vocabulary. It is retained in the corpus for completeness, though it is not directly cited in the currently published volumes of Panta Rhei.