diff --git a/preserves-schema.md b/preserves-schema.md
index 79b0bc9..48527b6 100644
--- a/preserves-schema.md
+++ b/preserves-schema.md
@@ -4,7 +4,7 @@ title: "Preserves Schema"
 ---
 
 Tony Garnock-Jones <tonyg@leastfixedpoint.com>  
-December 2023. Version 0.3.5.
+April 2024. Version 0.4.
 
   [abnf]: https://tools.ietf.org/html/rfc7405
   [identifierlike]: #sufficiently-identifierlike-values
@@ -415,6 +415,8 @@ entry.
 
     DictionaryPattern = "{" *(value ":" NamedSimplePattern) "}"
 
+**Extensibility.** All of these compound patterns allow for extensibility in that they allow additional fields, elements, or dictionary entries, respectively. For example, the pattern `<a @value int>` will match not only `<a 123>` but also `<a 123 "hello">`, while rejecting `<a>` or `<a [x y z]>`. Similarly, `{a: int, b: int}` matches `{a: 123, b: 234, c: [x y z]}` as well as `{a: 123, b: 234}`. Each compound pattern places constraints only on the *mentioned* elements of a datum, namely the leftmost fields of a record, the leftmost elements of a tuple, and the mentioned keys of a dictionary. Unmentioned elements are free to be present or absent.
+
 ### Identifiers and Bindings: NamedPattern and NamedSimplePattern
 
 Compound patterns specifications contain `NamedPattern`s or