diff --git a/Processors/6502Mk2/6502Mk2.hpp b/Processors/6502Mk2/6502Mk2.hpp
index c3841e13e..84d00a1d6 100644
--- a/Processors/6502Mk2/6502Mk2.hpp
+++ b/Processors/6502Mk2/6502Mk2.hpp
@@ -224,7 +224,8 @@ private:
 	int resume_point_ = FetchDecodeExecuteResumePoint;
 };
 
-template <Model model> class Processor;
+// TODO: concept to explain and verify Traits.
+template <Model model, typename Traits> class Processor;
 
 // MARK: - Implementations.