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#+title: Experiments with Realizability in Univalent Type Theory
#+author: Rahul Chhabra

This repository contains code for a formalisation of categorical realisability in cubical type theory.

This project is very much a work-in-progress and undergoing active hackery. As of right now, Agda will not type-check things with `--safe` enabled.

Here you can find both a timeline and an agenda of the formalisation. Some things are only formalised to the extent necessary.

* Combinatory Algebras
+ [X] Applicative Structures
+ [X] Feferman structure on an AS
+ [X] Combinatorial completeness
+ [X] Computation rule for $\lambda*$
+ Combinators
+ [X] Identity, booleans, if-then-else, pairs, projections, B combinator, some Curry numerals
+ [X] Computation rule for pairs
+ [ ] Fixpoint combinators and primitive recursion combinator
* Category of Assemblies
** Definition, limits and colimits
- [X] Define assemblies
- [X] Define the category $\mathsf{Asm}$
- [X] Cartesian closure and similar structure
- [X] Binary products
- [X] Binary coproducts
- [X] Equalisers
- [X] Exponentials
- [X] Initial and terminal objects
- [X] Coequalisers

** Regular and exact
- [ ] $\mathsf{Asm}$ is regular (requires Axiom of Choice)
- [x] Kernel pairs of morphisms exist
- [x] Kernel pairs have coequalisers
- [ ] Regular epics stable under pullback
- [ ] Exact completion
- [x] Internal equivalence relations of a category
- [ ] Functional relations

* Tripos to Topos Construction
See PR #6
** A valued Predicates
- [X] Heyting-valued Predicates
- [X] $\forall$ and $\exists$ are adjoints
- [X] Beck-Chevalley condition
- [X] Heyting prealgebra structure
- [X] Interpret intuitionistic logic
** Realisability Topos
+ [X] Partial Equivalence Relations
+ [X] Functional Relations
+ [X] Morphisms and RT is a category
+ [X] Finite limits
+ [X] Terminal object
+ [X] Binary products
+ [X] Equalisers (can be shown to merely exist)
+ [X] Power objects
+ [-] Monomorphisms
+ [ ] Subobjects and pullbacks lemmas
+ [ ] Power objects exist

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