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-rw-r--r--src/ContractionState.cxx1
1 files changed, 1 insertions, 0 deletions
diff --git a/src/ContractionState.cxx b/src/ContractionState.cxx
index af1c88bb7..551879c0c 100644
--- a/src/ContractionState.cxx
+++ b/src/ContractionState.cxx
@@ -12,6 +12,7 @@
#include <stdexcept>
#include <string_view>
#include <vector>
+#include <optional>
#include <algorithm>
#include <memory>