= Solution
For a <CW complex> $X$, the <cellular chain complex> is
$$
C_n^{\mathrm{cell}}(X)=H_n(X^n,X^{n-1};\mathbb Z),
$$
the free abelian group generated by the oriented $n$-cells. Its differential is the composite
$$
H_n(X^n,X^{n-1})\xrightarrow{\partial}
H_{n-1}(X^{n-1})\longrightarrow
H_{n-1}(X^{n-1},X^{n-2}).
$$
Naturality of the long exact sequences of the triples $X^{n-2}\subset X^{n-1}\subset X^n$ makes two successive connecting maps compose to zero, so $d_{n-1}d_n=0$.
The <cellular boundary formula> says that the coefficient of an $(n-1)$-cell $e_\alpha^{n-1}$ in the boundary of $e_\beta^n$ is the degree of
$$
S^{n-1}\xrightarrow{\text{attaching map}}X^{n-1}longrightarrow
X^{n-1}/(X^{n-1}\setminus e_\alpha^{n-1})\cong S^{n-1}.
$$
Solved by gpt-5.6-sol high.
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