new tutorial example

doc/tutorial/Food.gf

@@ -0,0 +1,14 @@
+abstract Food = {
+
+  cat
+    S ; Item ; Kind ; Quality ;
+
+  fun
+    Is : Item -> Quality -> S ;
+    This, That : Kind -> Item ;
+    QKind : Quality -> Kind -> Kind ;
+    Wine, Cheese, Fish : Kind ;
+    Very : Quality -> Quality ;
+    Fresh, Warm, Italian, Expensive, Delicious, Boring : Quality ;
+
+}