 Fiber product of groups (source code)

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= Fiber product of groups

For homomorphisms $E\to H$ and $G\to H$, their fiber product is
$$
E\times_HG=\{(e,g)\in E\times G:\pi(e)=f(g)\}.
$$
It is a subgroup of $E\times G$. Pulling a <group extension> back along $G\to H$ uses this fiber product as its middle group.

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