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Resizing rules for removing type‐in‐type

ReimannJ edited this page Aug 8, 2023 · 3 revisions

Summary of various attempts to introduce resizing rules to get rid of -type-in-type

RR1: $\frac{\Gamma\vdash is: isaprop X}{\Gamma\vdash X:Set}$ $\frac{(U_j,U_K:Univ)(X:U_j) (k < j) (is: isaprop X)}{X:U_k}$ $\frac{(U_j,U_K:Univ)(X:U_j) (k\le j) (is: isaprop X)}{X:U_k}$
RR2: $\frac{U:Univ}{\vdash (hProp U):Set}$ proposed by Voevodsky
RR2: $\frac{U:Univ}{\vdash (hProp U):U_1}, U_1$ above $Set$ allowing for set quotients to be in $Set$
make_hprop 1 $\frac{(U_j, U_k, U_l:Univ)(j < k)(j < l)(X: U_k)(isaprop X)}{\vdash X:((hProp U_j):U_l)}$
make_hprop 2 $\frac{(U_j, U_k, U_l:Univ)(k\le l)(j < l)(X: U_k)(isaprop X)}{\vdash X:((hProp U_j):U_l)}$ should allow for hprops of arbitrary sizes

with definition of hprop as $hprop:= \sum_{X: U_k} isaprop(X): U_l$ for $k < l$ ($k\le l$ caused Coq to complain as it allows for Type@{l} : Type@{l})

Another approach would be to add a resizing axiom instead of resizing rules, as for example done in the HoTT Library.