Add support for printing constants

Exercises/exercise-7/Lambda.hs

@@ -1,3 +1,4 @@
+import Data.Set (Set)
 --------------------
 -- Slides
 --------------------
@@ -14,6 +15,7 @@ pretty x = case x of
     (Inv func param) -> "(" ++ pretty func ++ " " ++ pretty param ++ ")"
     (Var name) -> name
     Add -> "Add"
+    Nat value -> show value
 
 -- example
 -- >>> pretty (Abs "x" $ Abs "y" $ Inv (Inv Add (Var "x")) (Var "y"))