Prove that the 'join' transformation is monotonic

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

Language.agda

@@ -91,3 +91,15 @@ record Program : Set where
 
     _≟_ : IsDecidable (_≡_ {_} {State})
     _≟_ = _≟ᶠ_
+
+    -- Computations for incoming and outgoing edged  will have to change too
+    -- when we support branching etc.
+
+    incoming : State → List State
+    incoming
+        with length
+    ...   | 0 = (λ ())
+    ...   | suc n' = (λ
+            { zero → []
+            ; (suc f') → (inject₁ f') ∷ []
+            })