diff --git a/pan/legacy.pan b/pan/legacy.pan
new file mode 100644
index 000000000..8a94ae326
--- /dev/null
+++ b/pan/legacy.pan
@@ -0,0 +1,3 @@
+declaration template pan/legacy;
+
+type legacy_binary_affirmation_string = string with match(SELF, "^(yes|no)$");
diff --git a/pan/types.pan b/pan/types.pan
index 68158a217..1ba6076c0 100644
--- a/pan/types.pan
+++ b/pan/types.pan
@@ -31,6 +31,7 @@ Data type and function definitions for the following basic types:
 
 declaration template pan/types;
 
+include 'pan/legacy';
 
 @documentation{
 This type implements a date/time format consistent with