diff --git a/README.md b/README.md
index 49213b6..2ab4295 100644
--- a/README.md
+++ b/README.md
@@ -1080,7 +1080,7 @@ Each of the individual types named in the union type must be unique.
     | }
     ? bad union type
 
-Cannot promote a union type to itself.
+One can, vacuously, promote a union type to itself.
 
     | fun main() {
     |   a = 20;
@@ -1089,20 +1089,20 @@ Cannot promote a union type to itself.
     |   d = c as integer|string
     |   print("ok")
     | }
-    ? bad cast
+    = ok
 
-Can promote a union type to another union type, so long as it is a superset.  (TODO)
+One can promote a union type to another union type, so long as it is a superset.
 
-    /| fun main() {
-    /|   a = 20;
-    /|   b = 30;
-    /|   c = a + b as integer|string
-    /|   d = c as integer|string|void
-    /|   print("ok")
-    /| }
-    /= ok
+    | fun main() {
+    |   a = 20;
+    |   b = 30;
+    |   c = a + b as integer|string
+    |   d = c as integer|string|void
+    |   print("ok")
+    | }
+    = ok
 
-Cannot promote a union type to a union type that is not a superset.
+One cannot promote a union type to a union type that is not a superset.
 
     | fun main() {
     |   a = 20;
diff --git a/TODO.md b/TODO.md
index fbf0345..4c4cdde 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,8 +30,6 @@ Other backends (Python? Java? CIL? Scheme?)
 
 ### Design ###
 
-Promote union type to bigger union type (e.g. `string|int` => `string|int|void`)
-
 Don't output final value.  Command-line arguments passed to `main`.  (`sysmain`?)
 
 Convenience:
diff --git a/src/castile/checker.py b/src/castile/checker.py
index 6870304..e756161 100644
--- a/src/castile/checker.py
+++ b/src/castile/checker.py
@@ -302,15 +302,15 @@ def type_of(self, ast):
         elif ast.tag == 'StructDefn':
             ast.type = Void()
         elif ast.tag == 'TypeCast':
-            val_t = self.type_of(ast.children[0])
-            uni_t = self.type_of(ast.children[1])
-            if not isinstance(uni_t, Union):
-                raise CastileTypeError('bad cast, not a union: %s' % uni_t)
-            if not uni_t.contains(val_t):
+            value_t = self.type_of(ast.children[0])
+            union_t = self.type_of(ast.children[1])
+            if not isinstance(union_t, Union):
+                raise CastileTypeError('bad cast, not a union: %s' % union_t)
+            if not union_t.contains(value_t):
                 raise CastileTypeError(
-                    'bad cast, %s does not include %s' % (uni_t, val_t)
+                    'bad cast, %s does not include %s' % (union_t, value_t)
                 )
-            ast.type = uni_t
+            ast.type = union_t
         else:
             raise NotImplementedError(repr(ast))
         return ast.type
diff --git a/src/castile/types.py b/src/castile/types.py
index 93c4c36..5be3891 100644
--- a/src/castile/types.py
+++ b/src/castile/types.py
@@ -53,9 +53,11 @@ class Union(Type):
     def __init__(self, content_types):
         self.content_types = content_types
 
-    def contains(self, type):
+    def contains(self, type_):
+        if isinstance(type_, Union):
+            return all([self.contains(t) for t in type_.content_types])
         for member in self.content_types:
-            if type == member:
+            if type_ == member:
                 return True
         return False