diff --git a/include/fcarouge/kalman.hpp b/include/fcarouge/kalman.hpp
index b52396ba5..f4b94ee0f 100644
--- a/include/fcarouge/kalman.hpp
+++ b/include/fcarouge/kalman.hpp
@@ -43,40 +43,12 @@ For more information, please refer to <https://unlicense.org> */
 //! @brief The main Kalman filter class.
 
 #include "kalman_equation.hpp"
+#include "kalman_operator.hpp"
 
 #include <type_traits>
 
 namespace fcarouge
 {
-template <typename Type> struct transpose {
-  [[nodiscard]] inline constexpr auto operator()(const Type &value)
-  {
-    return value;
-  }
-};
-
-template <typename Type> struct symmetrize {
-  [[nodiscard]] inline constexpr auto operator()(const Type &value)
-  {
-    return value;
-  }
-};
-
-template <typename Numerator, typename Denominator> struct divide {
-  [[nodiscard]] inline constexpr auto operator()(const Numerator &numerator,
-                                                 const Denominator &denominator)
-  {
-    return numerator / denominator;
-  }
-};
-
-template <typename Type> struct identity {
-  [[nodiscard]] inline constexpr Type operator()()
-  {
-    return 1;
-  }
-};
-
 //! @brief Kalman filter.
 //!
 //! @tparam State The type template parameter of the state vector x.
diff --git a/include/fcarouge/kalman_operator.hpp b/include/fcarouge/kalman_operator.hpp
new file mode 100644
index 000000000..27283b843
--- /dev/null
+++ b/include/fcarouge/kalman_operator.hpp
@@ -0,0 +1,78 @@
+/*_  __          _      __  __          _   _
+ | |/ /    /\   | |    |  \/  |   /\   | \ | |
+ | ' /    /  \  | |    | \  / |  /  \  |  \| |
+ |  <    / /\ \ | |    | |\/| | / /\ \ | . ` |
+ | . \  / ____ \| |____| |  | |/ ____ \| |\  |
+ |_|\_\/_/    \_\______|_|  |_/_/    \_\_| \_|
+
+Kalman Filter for C++
+Version 0.1.0
+https://github.com/FrancoisCarouge/Kalman
+
+SPDX-License-Identifier: Unlicense
+
+This is free and unencumbered software released into the public domain.
+
+Anyone is free to copy, modify, publish, use, compile, sell, or
+distribute this software, either in source code form or as a compiled
+binary, for any purpose, commercial or non-commercial, and by any
+means.
+
+In jurisdictions that recognize copyright laws, the author or authors
+of this software dedicate any and all copyright interest in the
+software to the public domain. We make this dedication for the benefit
+of the public at large and to the detriment of our heirs and
+successors. We intend this dedication to be an overt act of
+relinquishment in perpetuity of all present and future rights to this
+software under copyright law.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.
+
+For more information, please refer to <https://unlicense.org> */
+
+#ifndef FCAROUGE_KALMAN_OPERATOR_HPP
+#define FCAROUGE_KALMAN_OPERATOR_HPP
+
+//! @file
+//! @brief Kalman operation for standard types.
+
+namespace fcarouge
+{
+template <typename Type> struct transpose {
+  [[nodiscard]] inline constexpr auto operator()(const Type &value)
+  {
+    return value;
+  }
+};
+
+template <typename Type> struct symmetrize {
+  [[nodiscard]] inline constexpr auto operator()(const Type &value)
+  {
+    return value;
+  }
+};
+
+template <typename Numerator, typename Denominator> struct divide {
+  [[nodiscard]] inline constexpr auto operator()(const Numerator &numerator,
+                                                 const Denominator &denominator)
+  {
+    return numerator / denominator;
+  }
+};
+
+template <typename Type> struct identity {
+  [[nodiscard]] inline constexpr Type operator()()
+  {
+    return 1;
+  }
+};
+
+} // namespace fcarouge
+
+#endif // FCAROUGE_KALMAN_OPERATOR_HPP
diff --git a/include/fcarouge/eigen.hpp b/include/fcarouge/kalman_operator_eigen.hpp
similarity index 94%
rename from include/fcarouge/eigen.hpp
rename to include/fcarouge/kalman_operator_eigen.hpp
index fd5e3d4a6..90ca2b63c 100644
--- a/include/fcarouge/eigen.hpp
+++ b/include/fcarouge/kalman_operator_eigen.hpp
@@ -36,8 +36,11 @@ OTHER DEALINGS IN THE SOFTWARE.
 
 For more information, please refer to <https://unlicense.org> */
 
-#ifndef FCAROUGE_EIGEN_HPP
-#define FCAROUGE_EIGEN_HPP
+#ifndef FCAROUGE_KALMAN_OPERATOR_EIGEN_HPP
+#define FCAROUGE_KALMAN_OPERATOR_EIGEN_HPP
+
+//! @file
+//! @brief Kalman operation for Eigen 3 types.
 
 #include "kalman.hpp"
 
@@ -95,4 +98,4 @@ using kalman =
 
 } // namespace fcarouge::eigen
 
-#endif // FCAROUGE_EIGEN_HPP
+#endif // FCAROUGE_KALMAN_OPERATOR_EIGEN_HPP
diff --git a/test/test.cpp b/test/test.cpp
index 13979fbd4..e3f4059f6 100644
--- a/test/test.cpp
+++ b/test/test.cpp
@@ -36,9 +36,6 @@ OTHER DEALINGS IN THE SOFTWARE.
 
 For more information, please refer to <https://unlicense.org> */
 
-#include "fcarouge/eigen.hpp"
-#include "fcarouge/kalman.hpp"
-
 #include <cassert>
 
 namespace fcarouge::test