Use records rather than nested pairs to represent 'fixed height'

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

Analysis/Forward.agda

@@ -19,6 +19,7 @@ open import Relation.Binary.PropositionalEquality using (_≡_; refl; sym; trans
 open import Relation.Nullary using (¬_; Dec; yes; no)
 open import Data.Unit using (⊤)
 open import Function using (_∘_; flip)
+import Chain
 
 open import Utils using (Pairwise; _⇒_; _∨_)
 import Lattice.FiniteValueMap
@@ -77,7 +78,7 @@ module WithProg (prog : Program) where
     ≈ᵛ-dec = ≈ˡ-dec⇒≈ᵛ-dec ≈ˡ-dec
     joinSemilatticeᵛ = IsFiniteHeightLattice.joinSemilattice isFiniteHeightLatticeᵛ
     fixedHeightᵛ = IsFiniteHeightLattice.fixedHeight isFiniteHeightLatticeᵛ
-    ⊥ᵛ = proj₁ (proj₁ (proj₁ fixedHeightᵛ))
+    ⊥ᵛ = Chain.Height.⊥ fixedHeightᵛ
 
     -- Finally, the map we care about is (state -> (variables -> value)). Bring that in.
     module StateVariablesFiniteMap = Lattice.FiniteValueMap.WithKeys _≟_ isLatticeᵛ states