tidy up

include/evalhyd/evald.hpp

@@ -3,7 +3,6 @@
 
 #include <unordered_map>
 #include <vector>
-#include <optional>
 
 #include <xtensor/xexpression.hpp>
 #include <xtensor/xtensor.hpp>
@@ -160,6 +159,7 @@ namespace evalhyd
             const std::vector<std::string>& dts = {}
     )
     {
+        // retrieve real types of the expressions
         const D2& q_obs_ = q_obs.derived_cast();
         const D2& q_prd_ = q_prd.derived_cast();