diff --git a/WoofWare.Whippet.Fantomas/SurfaceBaseline.txt b/WoofWare.Whippet.Fantomas/SurfaceBaseline.txt
index 9475898..9c63951 100644
--- a/WoofWare.Whippet.Fantomas/SurfaceBaseline.txt
+++ b/WoofWare.Whippet.Fantomas/SurfaceBaseline.txt
@@ -271,6 +271,7 @@ WoofWare.Whippet.Fantomas.SynField.extractWithIdent [static method]: Fantomas.FC
 WoofWare.Whippet.Fantomas.SynField.make [static method]: Fantomas.FCS.Syntax.Ident option WoofWare.Whippet.Fantomas.SynFieldData -> Fantomas.FCS.Syntax.SynField
 WoofWare.Whippet.Fantomas.SynField.mapIdent [static method]: ('a -> 'b) -> 'a WoofWare.Whippet.Fantomas.SynFieldData -> 'b WoofWare.Whippet.Fantomas.SynFieldData
 WoofWare.Whippet.Fantomas.SynField.withDocString [static method]: Fantomas.FCS.Xml.PreXmlDoc -> Fantomas.FCS.Syntax.SynField -> Fantomas.FCS.Syntax.SynField
+WoofWare.Whippet.Fantomas.SynField.withMutability [static method]: bool -> Fantomas.FCS.Syntax.SynField -> Fantomas.FCS.Syntax.SynField
 WoofWare.Whippet.Fantomas.SynFieldData`1 inherit obj
 WoofWare.Whippet.Fantomas.SynFieldData`1..ctor [constructor]: (Fantomas.FCS.Syntax.SynAttribute list, 'Ident, Fantomas.FCS.Syntax.SynType)
 WoofWare.Whippet.Fantomas.SynFieldData`1.Attrs [property]: [read-only] Fantomas.FCS.Syntax.SynAttribute list
diff --git a/WoofWare.Whippet.Fantomas/SynField.fs b/WoofWare.Whippet.Fantomas/SynField.fs
index 4c927d1..eccfaaa 100644
--- a/WoofWare.Whippet.Fantomas/SynField.fs
+++ b/WoofWare.Whippet.Fantomas/SynField.fs
@@ -78,3 +78,10 @@ module SynField =
         match f with
         | SynField (attributes, isStatic, idOpt, fieldType, isMutable, _, accessibility, range, trivia) ->
             SynField (attributes, isStatic, idOpt, fieldType, isMutable, doc, accessibility, range, trivia)
+
+    /// Set the mutability of this `SynField`.
+    /// For example, `{ mutable Foo : int }` is a record with one mutable field.
+    let withMutability (isMutable : bool) (f : SynField) : SynField =
+        match f with
+        | SynField (attributes, isStatic, idOpt, fieldType, _, doc, accessibility, range, trivia) ->
+            SynField (attributes, isStatic, idOpt, fieldType, isMutable, doc, accessibility, range, trivia)
diff --git a/WoofWare.Whippet.Fantomas/version.json b/WoofWare.Whippet.Fantomas/version.json
index a6bbf9b..59bb1bf 100644
--- a/WoofWare.Whippet.Fantomas/version.json
+++ b/WoofWare.Whippet.Fantomas/version.json
@@ -1,5 +1,5 @@
 {
-  "version": "0.4",
+  "version": "0.5",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],
@@ -9,4 +9,4 @@
     ":/global.json",
     ":/Directory.Build.props"
   ]
-}
+}
\ No newline at end of file