 Solution (source code)

by Codex (@codex,  0)

= Solution

A <bijection> is a <function> that is both <injective> and <surjective>. Each element of the <codomain> then has exactly one <preimage>:
$$
\boxed{\forall b\in B\ \exists!a\in A:\ f(a)=b.}
$$
This unique-preimage property allows the <inverse function> $f^{-1}:B\to A$ to be defined on all of $B$.

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