Documentation

GroupoidModel.Russell_PER_MS.UHom

Morphisms of natural models, and Russell-universe embeddings.

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    def NaturalModelBase.Hom.comp {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] {M N O : NaturalModelBase Ctx} (α : M.Hom N) (β : N.Hom O) :
    M.Hom O
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      def NaturalModelBase.Hom.comp_assoc {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] {M N O P : NaturalModelBase Ctx} (α : M.Hom N) (β : N.Hom O) (γ : O.Hom P) :
      (α.comp β).comp γ = α.comp (β.comp γ)
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        Morphism into the representable natural transformation M from the pullback of M along a type.

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          Given M : NaturalModelBase, a semantic type A : y(Γ) ⟶ M.Ty, and a substitution σ : Δ ⟶ Γ, construct a Hom for the substitution A[σ].

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            A Russell embedding is a hom of natural models M ⟶ N such that types in M correspond to terms of a universe U in N.

            These don't form a category since UHom.id M is essentially Type : Type in M.

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                  def NaturalModelBase.UHom.comp_assoc {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] {M N O P : NaturalModelBase Ctx} (α : M.UHom N) (β : N.UHom O) (γ : O.UHom P) :
                  (α.comp β).comp γ = α.comp (β.comp γ)
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                      A sequence of Russell embeddings.

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                          @[irreducible]
                          def NaturalModelBase.UHomSeq.hom {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeq Ctx) (i j : ) (ij : i < j := by omega) (jlen : j < s.length + 1 := by get_elem_tactic) :

                          Composition of embeddings between i and j in the chain.

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                            @[irreducible]
                            theorem NaturalModelBase.UHomSeq.hom_comp_trans {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeq Ctx) (i j k : ) (ij : i < j) (jk : j < k) (klen : k < s.length + 1) :
                            (s.hom i j ij ).comp (s.hom j k jk klen) = s.hom i k klen
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                                The data of Π and λ term formers for every i, j ≤ length + 1, interpreting

                                Γ ⊢ᵢ A type  Γ.A ⊢ⱼ B type
                                --------------------------
                                Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ ΠA. B type
                                

                                and

                                Γ ⊢ᵢ A type  Γ.A ⊢ⱼ t : B
                                -------------------------
                                Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ λA. t : ΠA. B
                                

                                This amounts to O(length²) data. One might object that the same formation rules could be modeled with O(length) data: etc etc

                                However, with O(length²) data we can use Lean's own type formers directly, rather than using Π (ULift A) (ULift B). The interpretations of types are thus more direct.

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                                        def NaturalModelBase.UHomSeqPis.Pi_pbs {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeqPis Ctx) (i : ) (ilen : i < s.length + 1 := by get_elem_tactic) :
                                        CategoryTheory.IsPullback (s.lams i ilen) (s[i].Ptp.map s[i].tp) s[i].tp (s.Pis i ilen)
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                                              Γ ⊢ᵢ A  Γ.A ⊢ⱼ B
                                              -----------------
                                              Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ ΠA. B
                                              
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                                                Γ ⊢ᵢ A  Γ.A ⊢ⱼ t : B
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                                                Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ λA. t : ΠA. B
                                                
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                                                  Γ ⊢ᵢ A  Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ f : ΠA. B
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                                                  Γ.A ⊢ⱼ unlam f : B
                                                  
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                                                    Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ f : ΠA. B  Γ ⊢ᵢ a : A
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                                                    Γ ⊢ⱼ f a : B[id.a]
                                                    
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                                                      theorem NaturalModelBase.UHomSeqPis.mkLam_unLam {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeqPis Ctx) {i j : } (ilen : i < s.length + 1) (jlen : j < s.length + 1) {Γ : Ctx} (A : CategoryTheory.yoneda.obj Γ s[i].Ty) (B : CategoryTheory.yoneda.obj (s[i].ext A) s[j].Ty) (f : CategoryTheory.yoneda.obj Γ s[i j].Tm) (f_tp : CategoryTheory.CategoryStruct.comp f s[i j].tp = s.mkPi ilen jlen A B) :
                                                      s.mkLam ilen jlen A (s.unLam ilen jlen A B f f_tp) = f
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                                                      theorem NaturalModelBase.UHomSeqPis.unLam_mkLam {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeqPis Ctx) {i j : } (ilen : i < s.length + 1) (jlen : j < s.length + 1) {Γ : Ctx} (A : CategoryTheory.yoneda.obj Γ s[i].Ty) (B : CategoryTheory.yoneda.obj (s[i].ext A) s[j].Ty) (t : CategoryTheory.yoneda.obj (s[i].ext A) s[j].Tm) (t_tp : CategoryTheory.CategoryStruct.comp t s[j].tp = B) (lam_tp : CategoryTheory.CategoryStruct.comp (s.mkLam ilen jlen A t) s[i j].tp = s.mkPi ilen jlen A B) :
                                                      s.unLam ilen jlen A B (s.mkLam ilen jlen A t) lam_tp = t
                                                      Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ f : ΠA. B
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                                                      Γ ⊢ₘₐₓ₍ᵢ,ⱼ₎ λA. f[↑] v₀ : ΠA. B
                                                      
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                                                        theorem NaturalModelBase.UHomSeqPis.mkApp_mkLam {Ctx : Type u} [CategoryTheory.SmallCategory Ctx] [CategoryTheory.ChosenFiniteProducts Ctx] (s : UHomSeqPis Ctx) {i j : } (ilen : i < s.length + 1) (jlen : j < s.length + 1) {Γ : Ctx} (A : CategoryTheory.yoneda.obj Γ s[i].Ty) (B : CategoryTheory.yoneda.obj (s[i].ext A) s[j].Ty) (t : CategoryTheory.yoneda.obj (s[i].ext A) s[j].Tm) (t_tp : CategoryTheory.CategoryStruct.comp t s[j].tp = B) (lam_tp : CategoryTheory.CategoryStruct.comp (s.mkLam ilen jlen A t) s[i j].tp = s.mkPi ilen jlen A B) (a : CategoryTheory.yoneda.obj Γ s[i].Tm) (a_tp : CategoryTheory.CategoryStruct.comp a s[i].tp = A) :
                                                        s.mkApp ilen jlen A B (s.mkLam ilen jlen A t) lam_tp a a_tp = s[i].inst A t a a_tp
                                                        Γ ⊢ᵢ A  Γ.A ⊢ⱼ t : B  Γ ⊢ᵢ a : A
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                                                        Γ.A ⊢ⱼ (λA. t) a ≡ t[a] : B[a]