1 files changed, 3 insertions, 0 deletions
diff --git a/src/mask.h b/src/mask.h
index 8c20e554..755a2229 100644
--- a/src/mask.h
+++ b/src/mask.h
@@ -74,6 +74,9 @@ public:
 
   mask_t& operator=(const string& other);
 
+  bool operator<(const mask_t& other) const {
+    return expr < other.expr;
+  }
   bool operator==(const mask_t& other) const {
     return expr == other.expr;
   }