diff --git a/src/ast_values.hpp b/src/ast_values.hpp
index b2f2c3a..b79ac84 100644
--- a/src/ast_values.hpp
+++ b/src/ast_values.hpp
@@ -14,6 +14,7 @@
     enum_();
     enum_(const std::string& value);
     bool operator==(const enum_& b) const;
+    bool operator<(const enum_& b) const;
     std::string m_value;
 };
 
@@ -21,11 +22,13 @@
     binary_();
     binary_(const std::string& value);
     bool operator==(const binary_& b) const;
+    bool operator<(const binary_& b) const;
     std::string m_value;
 };
 
 struct module_ {
     bool operator==(const module_& b) const;
+    bool operator<(const module_& b) const;
     std::string m_name;
 };
 
@@ -34,6 +37,7 @@
     identityRef_(const std::string& module, const std::string& value);
     identityRef_(const std::string& value);
     bool operator==(const identityRef_& b) const;
+    bool operator<(const identityRef_& b) const;
     boost::optional<module_> m_prefix;
     std::string m_value;
 };
@@ -46,6 +50,7 @@
 
 struct special_ {
     bool operator==(const special_& b) const;
+    bool operator<(const special_& b) const;
     SpecialValue m_value;
 };