Make 'IsDecidable' into a record to aid instance search

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

Lattice/AboveBelow.agda

@@ -5,13 +5,14 @@ open import Relation.Nullary using (Dec; ¬_; yes; no)
 module Lattice.AboveBelow {a} (A : Set a)
                           (_≈₁_ : A → A → Set a)
                           (≈₁-equiv : IsEquivalence A _≈₁_)
-                          (≈₁-dec : IsDecidable _≈₁_) where
+                          (≈₁-Decidable : IsDecidable _≈₁_) where
 
 open import Data.Empty using (⊥-elim)
 open import Data.Product using (_,_)
 open import Data.Nat using (_≤_; ℕ; z≤n; s≤s; suc)
 open import Function using (_∘_)
 open import Showable using (Showable; show)
+open import Relation.Binary.Definitions using (Decidable)
 open import Relation.Binary.PropositionalEquality as Eq
     using (_≡_; sym; subst; refl)
 
@@ -20,6 +21,8 @@ import Chain
 open IsEquivalence ≈₁-equiv using ()
     renaming (≈-refl to ≈₁-refl; ≈-sym to ≈₁-sym; ≈-trans to ≈₁-trans)
 
+open IsDecidable ≈₁-Decidable using () renaming (R-dec to ≈₁-dec)
+
 data AboveBelow : Set a where
     ⊥ : AboveBelow
     ⊤ : AboveBelow
@@ -62,7 +65,7 @@ data _≈_ : AboveBelow → AboveBelow → Set a where
     ; ≈-trans = ≈-trans
     }
 
-≈-dec : IsDecidable _≈_
+≈-dec : Decidable _≈_
 ≈-dec ⊥ ⊥ = yes ≈-⊥-⊥
 ≈-dec ⊤ ⊤ = yes ≈-⊤-⊤
 ≈-dec [ x ] [ y ]
@@ -76,6 +79,9 @@ data _≈_ : AboveBelow → AboveBelow → Set a where
 ≈-dec [ x ] ⊥ = no λ ()
 ≈-dec [ x ] ⊤ = no λ ()
 
+≈-Decidable : IsDecidable _≈_
+≈-Decidable = record { R-dec = ≈-dec }
+
 -- Any object can be wrapped in an 'above below' to make it a lattice,
 -- since ⊤ and ⊥ are the largest and least elements, and the rest are left
 -- unordered. That's what this module does.