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+{-# OPTIONS --without-K --safe #-}
+
+open import Relation.Binary using (Setoid)
+
+module Cfe.Derivation.Base
+  {c ℓ} (over : Setoid c ℓ)
+  where
+
+open Setoid over renaming (Carrier to C; _≈_ to _∼_)
+
+open import Cfe.Expression over hiding (_≋_)
+open import Data.Fin
+open import Data.List
+open import Data.List.Relation.Binary.Equality.Setoid over
+open import Level using (_⊔_)
+
+infix 4 _⤇_
+
+data _⤇_ : Expression 0 → List C → Set (c ⊔ ℓ) where
+  Eps : ε ⤇ []
+  Char : ∀ {c y} → c ∼ y → Char c ⤇ [ y ]
+  Cat : ∀ {e₁ e₂ l₁ l₂ l} → e₁ ⤇ l₁ → e₂ ⤇ l₂ → l₁ ++ l₂ ≋ l → e₁ ∙ e₂ ⤇ l
+  Veeˡ : ∀ {e₁ e₂ l} → e₁ ⤇ l → e₁ ∨ e₂ ⤇ l
+  Veeʳ : ∀ {e₁ e₂ l} → e₂ ⤇ l → e₁ ∨ e₂ ⤇ l
+  Fix : ∀ {e l} → e [ μ e / zero ] ⤇ l → μ e ⤇ l