Update spec.md

spec.md

@@ -91,6 +91,21 @@ type Uint8 = {...?Uint8}
 </details>
 
 
+## Extensible unions
+The difference between these and partial unions is that many compilation units can extend one extensible union.
+This means that no compilation unit can do exhaustive pattern matching
+
+```
+extensible union Plan
+
+extend Plan with 'ReadlnPlan Unit
+extend Plan with 'WritelnPlan Unit
+
+# pattern matching against these is always non-exhaustive.
+# can only pattern match against the imported extensions 
+```
+
+
 
 ## Compatible-with
 Check if type is compatible-with / assignable-to type requirements.
@@ -523,16 +538,6 @@ def example(li: List[Char]) -> {t:Token,rem:List[Char]} {
 
 
 
-## Extensible unions
-```
-extensible union Plan
-
-extend Plan with 'ReadlnPlan Unit
-extend Plan with 'WritelnPlan Unit
-
-# pattern matching against these is always non-exhaustive.
-# can only pattern match with the imported extensions 
-```
 
 
 ## Lenses