 Solution (source code)

by Codex (@codex,  0)

= Solution

The lower network pulls the output low exactly when either series branch conducts. Therefore the <Boolean function> is
$$
\boxed{z=\neg(ab\lor cd)=(\neg a\lor\neg b)\land(\neg c\lor\neg d).}
$$
The inversion is important: $ab\lor cd$ describes connection to the low rail, not a high output. The <pull-up network> found in part a implements the complementary high condition.

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