diff --git a/README.md b/README.md
index 34628a5..4c77565 100644
--- a/README.md
+++ b/README.md
@@ -32,9 +32,12 @@ type Enclosure interface {
 func Enclose(e Enclosure, w ...Widget) Widget { ... }
 ```
 
-The `Context` struct also contains a `map[string]interface{}` allowing arbitrary
-data to be maintained. This can be used, for example, to define themes, and control
-the operation of a `WidgetCombinator` during operation. For example>
+The giowrap package also includes a private package-global variable called
+extra that is a []interface{}, allowing arbitrary data to be maintained by
+UI elements. Elements can request access to the structure when they are
+allocated and they are returned a unique integer index into the slice. This
+can be used, for example, to define themes, and control
+the operation of a `WidgetCombinator` during operation. For example:
 
 ```go
 type fWidget struct { l Layout }
@@ -42,7 +45,7 @@ func NewfWidget(l layout) fWidget { return fWidget{ l: l } }
 
 func Flexible(v float32) Widget {
 	return NewfWidget(func(ctx Context) Context {
-		ctx.extra["Flexible"] = v
+		extra.data[extra.cur].(*FlexOpts).flexible = v
 		return ctx
 	})
 }