v0.0.6: allow infinite, but not infinite sized types

spec.md

@@ -200,3 +200,18 @@ type List[t] = &a ('End | 'Cons {head:t, tail:a})
 # a List[List[t]] is just:
 &b ('End | 'Cons {head: &a ('End | 'Cons {head:t, tail:a}), tail: b})
 ```
+
+Infinitely sized types are not allowed:
+```
+&a {x:Num, y:a}
+```
+
+However, infinite types without size *are* allowed:
+```
+&a {x:a}
+```
+
+This is *not* allowed:
+```
+&a a
+```