diff --git a/src/ast_values.hpp b/src/ast_values.hpp
index f2d1c7e..9ec8389 100644
--- a/src/ast_values.hpp
+++ b/src/ast_values.hpp
@@ -26,6 +26,12 @@
     std::string m_value;
 };
 
+struct empty_ {
+    empty_();
+    bool operator==(const empty_) const;
+    bool operator<(const empty_) const;
+};
+
 struct module_ {
     bool operator==(const module_& b) const;
     bool operator<(const module_& b) const;
@@ -63,6 +69,7 @@
 
 using leaf_data_ = boost::variant<enum_,
                                   binary_,
+                                  empty_,
                                   identityRef_,
                                   special_,
                                   double,