diff --git a/src/CwF.lagda.md b/src/CwF.lagda.md index fe7fd45..4d9e975 100644 --- a/src/CwF.lagda.md +++ b/src/CwF.lagda.md @@ -41,9 +41,6 @@ open import Fams module CwF where ``` - # Categories with families A Category with families, written as CwF, is a category in which we can interpret the synax of a type theory. The objects of a CwF are contexts, and morphisms are a list of judgements, which capture the notions of substitution and weakening. For a more detailed introduction to CwFs see the introductory paper by Hoffmann. [@hoffmann97] @@ -128,19 +125,19 @@ For every context $\Gamma$ and $A ∈ Ty(\Gamma)$ there is a projection map $p_{ q : {Γ : Ob} {A : ∣ Ty Γ ∣} → ∣ Tr (Γ ; A) (A ⟦ p ⟧) ∣ ``` -The meaning of $q$ might be a bit confusing, but it's essentialy an inverse of $p$, in the sense made precise below. +It's required that the map $p$ weakened by $q$ is the identity. ``` field pq-id : {Γ : Ob} {A : ∣ Ty Γ ∣} → ⟨ p , q ⟩ ≡ id {Γ ; A} ``` -There are some additional requirments placed upon $p$ and $q$ to ensure they play nice. +Dubbing $p$ a projection is no coincidence, it projects a map out of a weakened map. ``` p-∘ : {Δ Γ : Ob} {γ : Hom Δ Γ} {A : ∣ Ty Γ ∣} (a : ∣ Tr Δ (A ⟦ γ ⟧) ∣) → p ∘ ⟨ γ , a ⟩ ≡ γ ``` -The condition for q, depending on the condition for p holding, needs to be expressed with a rather annoying path over path. +Likewise, $q$ "projects" an object out of a weakened substitution. ``` q-∘-pathp : {Δ Γ : Ob} {γ : Hom Δ Γ} {A : ∣ Ty Γ ∣} (a : ∣ Tr Δ (A ⟦ γ ⟧) ∣) → ∣ Tr Δ (F₁ ⟨ γ , a ⟩ .fst (F₁ p .fst A)) ∣ ≡ ∣ Tr Δ (F₁ γ .fst A) ∣