resource examples in tutorial

doc/tutorial/arithm/Arithm.gf

@@ -0,0 +1,13 @@
+abstract Arithm = {
+
+  cat
+    Prop ;
+    Nat ;
+
+  fun
+    Zero : Nat ;
+    Succ : Nat -> Nat ;
+    Even : Nat -> Prop ;
+    And  : Prop -> Prop -> Prop ;
+
+}