add a couple of failing examples

examples/fail.quox

@@ -0,0 +1,10 @@
+@[fail "but cases for"]
+def missing-b : {a, b} → {a} =
+  λ x ⇒ case x return {a} of { 'a ⇒ 'a }
+
+-- @[fail "duplicate tags"]
+-- def repeat-enum-case : {a} → {a} =
+--   λ x ⇒ case x return {a} of { 'a ⇒ 'a; 'a ⇒ 'a }
+
+@[fail "duplicate tags"]
+def repeat-enum-type : {a, a} = 'a