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Add equivalence between is-set and is-0-type - #185

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Add equivalence between is-set and is-0-type#185
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@marco-david

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In response to issue #175

Comment thread src/hott/12-sets.rzk.md
#def is-property-is-set uses (funext)
( A : U)
: is-prop (is-set A)
-- This proof was written by ChatGPT 5.6 Sol.

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I'm not sure we want to intersperse comments like this, nor if we want to have AI generated proofs in the library.
@fizruk @emilyriehl @jonweinb

Comment thread src/hott/12-sets.rzk.md
Next, we explicitly construct the equivalence between sets and 0-types.

```rzk
#def is-set-to-is-0-type

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Suggested change
#def is-set-to-is-0-type
#def is-0-type-is-set

Comment thread src/hott/12-sets.rzk.md
: is-set A → is-0-type A
:= \ s x y → is-prop-all-elements-equal (x = y) (s x y)

#def is-0-type-to-is-set

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Suggested change
#def is-0-type-to-is-set
#def is-set-is-0-type

Comment thread src/hott/12-sets.rzk.md
: is-0-type A → is-set A
:= \ t x y → all-elements-equal-is-prop (x = y) (t x y)

#def is-set-is-0-type uses (weakfunext funext)

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Suggested change
#def is-set-is-0-type uses (weakfunext funext)
#def equiv-is-set-is-0-type uses (weakfunext funext)

@fredrik-bakke fredrik-bakke linked an issue Aug 19, 2026 that may be closed by this pull request
@fredrik-bakke

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Hey @marco-david! I've spoken with the TAs about this now and we've decided we will leave these goals open for someone to formalize as an exercise in HoTT and to familiarize themselves with the library for now. It's not critical to have this merged, but if no one else does it we might merge your contribution later.

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Equivalent characterizations of sets

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