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Is there any point to having both type of propositions and Prop? The analogous page-name Type is a redirect to type of types.
Not sure why this happened. I have merged the two entries now.
Should be possible to define the type of propositions via the universal property of the subobject classifier, something like
$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \Omega \; \mathrm{type}}$ $\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathrm{true}:\Omega}$ $\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash i:A \hookrightarrow B}{\Gamma \vdash \chi_B:B \to \Omega}$et cetera
That’s effectively being done already when you say every proposition is classified by $\Omega$. To derive your proposed rule, consider the proposition in context $\Gamma.(b : B)$ of preimages of $b$ under $i$.
Side note: we don’t currently have models of the type of all propositions (as currently defined, with a meta-equality $\mathsf{El}(A_\Omega) \equiv A$) in HoTT.
Incidentally, I see that this entry is lacking any reference to subtype.
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