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+/-
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Ayberk Tosun
+-/
+
+module
+
+public import Cslib.Foundations.Semantics.LTS.Basic
+public import Mathlib.CategoryTheory.Category.Basic
+open Cslib
+
+universe u v
+
+@[expose] public section
+
+variable {State Label : Type}
+
+/-!
+# Category of Labelled Transition Systems
+-/
+
+/-! ## LTSs and LTS morphisms form a category -/
+
+/-- The notion of labelled transition system -/
+structure BundledLTS where
+  State : Type u
+  Label : Type v
+  lts : LTS State Label
+
+/- Remark: I do not like the name 'bundled LTS'; and LTS is already the bundled notion. The name
+   `LTS` for the transition relation on a fixed set of states and labels is what is confusing here.
+    I propose to change that to `LTS-Structure` and call the above `LTS`.
+-/
+
+/-! ## Definition of LTS morphism -/
+
+/--
+A morphism between two labelled transition systems consists of a function on
+states, a function on labels, and a proof that transitions are preserved.
+-/
+structure LTS.Morphism (lts₁ lts₂ : BundledLTS) : Type where
+  toFun : lts₁.State → lts₂.State
+  labelMap : lts₁.Label → lts₂.Label
+  fun_preserves_transitions : (s s' : lts₁.State)
+                            → (l : lts₁.Label)
+                            → lts₁.lts.Tr s l s'
+                            → lts₂.lts.Tr (toFun s) (labelMap l) (toFun s')
+
+/-- The identity LTS morphism. -/
+def LTS.Morphism.id (lts : BundledLTS) : LTS.Morphism lts lts :=
+  { toFun                     := _root_.id
+  , labelMap                  := _root_.id
+  , fun_preserves_transitions := fun _ _ _ h => h
+  }
+
+/-- Composition of LTS morphisms. -/
+def LTS.Morphism.comp {lts₁ lts₂ lts₃ : BundledLTS} :
+    LTS.Morphism lts₁ lts₂ → LTS.Morphism lts₂ lts₃ → LTS.Morphism lts₁ lts₃ :=
+  fun ⟨f, μ, p⟩ ⟨g, ν, q⟩ =>
+    let r := by intros _ _ _ h
+                apply q
+                apply p
+                exact h
+    ⟨g ∘ f, ν ∘ μ, r⟩
+
+/-- `LTS.Morphism` provides a category structure on bundled LTSs. -/
+instance : CategoryTheory.CategoryStruct BundledLTS where
+  Hom lts₁ lts₂         := LTS.Morphism lts₁ lts₂
+  id lts                := LTS.Morphism.id lts
+  comp {lts₁} {lts₂} {lts₃} f g := @LTS.Morphism.comp lts₁ lts₂ lts₃ f g
+
+/-- Proof that the above structure actually forms a category. -/
+instance : CategoryTheory.Category BundledLTS where
+  id_comp := by intro _ _ f
+                cases f
+                rfl
+  comp_id := by intro _ _ f
+                cases f
+                rfl
+  assoc := by intro _ _ _ _ f g h
+              cases f
+              cases g
+              cases h
+              rfl