--- src/dp/hmm.cc
+++ src/dp/hmm.cc	2024-05-28 15:23:45.000000000 +0800
@@ -24,6 +24,7 @@
 ///
 
 #include <cmath>
+#include <bitset>
 #include "hmm.H"
 #include "2way.H"
 #include "imodel/imodel.H"