OpenCV Code Style

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I am reading the source code of OpenCV under the guidance from The OpenCV Coding Style Guide. It says that the implementation is put to opencv/modules/<module_name>/src and interface is added to the header files in opencv/modules/<module_name>/include/opencv2/<module_name>. But I find some .hpp file under the opencv/modules/<module_name>/src folder, such as opencv/modules/imgproc/src/filterengine.hpp. So what kind of .hpp file should be put to this folder?