Convention 3.3.4. Throughout this section, we write \(D^n\) for the \(n\)-disk and \(S^{n-1}\) for its boundary sphere, viewed as animae. While contractibility of the disk implies that \(D^n \simeq \pt \), this notation emphasizes the analogy to cell attachments in CW-complexes. Given a map \(f\colon A \to X\) of animae, we write \(X/A\) for the cofiber of \(A_+\to X_+\).
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