diff --git a/src/mad.h b/src/mad.h
index 47d27be4..79257ab7 100644
--- a/src/mad.h
+++ b/src/mad.h
@@ -24,6 +24,7 @@ struct Chainable {
     virtual void set_zero_adjoint() { }
     
     static inline void* operator new(size_t nbytes) {
+      // thread_local variable
       return p.ordered_malloc(nbytes);
     }
 };