fix nat elim quantities

tests/Tests/Typechecker.idr

@@ -401,6 +401,23 @@ tests = "typechecker" :- [
     todo "nat elim"
   ],
 
+  "natural elim" :- [
+    note "1 · λ n ⇒ case1 n return ℕ of { zero ⇒ 0; succ n ⇒ n }",
+    note "    ⇐ 1.ℕ → ℕ",
+    testTC "pred" $
+      check_ empty sone
+        ([< "n"] :\\ E (CaseNat One Zero (BV 0) (SN Nat)
+            Zero (SY [< "n", Unused] $ BVT 1)))
+        (Arr One Nat Nat),
+    note "1 · λ m n ⇒ case1 m return ℕ of { zero ⇒ n; succ _, 1.p ⇒ succ p }",
+    note "    ⇐ 1.ℕ → 1.ℕ → 1.ℕ",
+    testTC "plus" $
+      check_ empty sone
+        ([< "m", "n"] :\\ E (CaseNat One One (BV 1) (SN Nat)
+          (BVT 0) (SY [< Unused, "p"] $ Succ $ BVT 0)))
+        (Arr One Nat $ Arr One Nat Nat)
+  ],
+
   "box types" :- [
     testTC "0 · [0.ℕ] ⇐ ★₀" $
       check_ empty szero (BOX Zero Nat) (TYPE 0),