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+{-# OPTIONS --without-K --safe #-}
+
+module Data.Fin.Preimage where
+
+open import Data.Nat.Base using (ℕ)
+open import Data.Fin.Base using (Fin)
+open import Function.Base using (∣_⟩-_; _∘_)
+open import Relation.Binary.PropositionalEquality as ≡ using (_≗_)
+open import Data.Subset.Functional using (Subset; foldl; _∪_; ⊥; ⁅_⁆)
+
+private
+  variable A B C : ℕ
+
+preimage : (Fin A → Fin B) → Subset B → Subset A
+preimage f Y = Y ∘ f
+
+⋃ : Subset A → (Fin A → Subset B) → Subset B
+⋃ S f = foldl (∣ _∪_ ⟩- f) ⊥ S
+
+image : (Fin A → Fin B) → Subset A → Subset B
+image f X = ⋃ X (⁅_⁆ ∘ f)
+
+preimage-cong₁
+    : {f g : Fin A → Fin B}
+    → f ≗ g
+    → (S : Subset B)
+    → preimage f S ≗ preimage g S
+preimage-cong₁ f≗g S x = ≡.cong S (f≗g x)
+
+preimage-cong₂
+    : (f : Fin A → Fin B)
+      {S₁ S₂ : Subset B}
+    → S₁ ≗ S₂
+    → preimage f S₁ ≗ preimage f S₂
+preimage-cong₂ f S₁≗S₂ = S₁≗S₂ ∘ f
+
+preimage-∘
+    : (f : Fin A → Fin B)
+      (g : Fin B → Fin C)
+      (z : Fin C)
+    → preimage (g ∘ f) ⁅ z ⁆ ≗ preimage f (preimage g ⁅ z ⁆)
+preimage-∘ f g S x = ≡.refl