diff --git a/spec.md b/spec.md
index 4eb8f9d..4ff2293 100644
--- a/spec.md
+++ b/spec.md
@@ -199,4 +199,19 @@ type List[t] = &a ('End | 'Cons {head:t, tail:a})
 # example 2:
 # 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
 ```
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