diff --git a/src/examples/string_guess.cc b/src/examples/string_guess.cc
new file mode 100644
index 0000000..fb69b24
--- /dev/null
+++ b/src/examples/string_guess.cc
@@ -0,0 +1,53 @@
+/*
+ *  \remark This file is part of ULTRA.
+ *
+ *  \copyright Copyright (C) 2024 EOS di Manlio Morini.
+ *
+ *  \license
+ *  This Source Code Form is subject to the terms of the Mozilla Public
+ *  License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ *  You can obtain one at http://mozilla.org/MPL/2.0/
+ *
+ *  \see https://github.com/morinim/ultra/wiki/string_guessing_tutorial
+ */
+
+#include <iostream>
+#include <string>
+
+#include "kernel/ultra.h"
+
+const std::string target = "Hello World";
+const std::string CHARSET =
+  " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!";
+
+// The fitness function.
+double fitness(const ultra::ga::individual &x)
+{
+  double found(0.0);
+
+  for (std::size_t i(0); i < target.length(); ++i)
+    if (target[i] == CHARSET[x[i]])
+      ++found;
+
+  return found;
+}
+
+int main()
+{
+  using namespace ultra;
+
+  // A solution of this problem is a fixed length (`target.length()`) string of
+  // characters in a given charset (`CHARSET`).
+  ga::problem prob(target.length(), {0, CHARSET.size()});
+
+  prob.params.population.individuals = 300;
+
+  ga::search search(prob, fitness);
+  auto result(search.run().best_individual);
+
+  std::cout << "\nBest result: ";
+  for (auto gene : result)
+    std::cout << CHARSET[gene];
+
+  std::cout << " (fitness " << fitness(result) << ")\n";
+}
diff --git a/src/kernel/ultra.h b/src/kernel/ultra.h
index ab12bfb..f87bf11 100644
--- a/src/kernel/ultra.h
+++ b/src/kernel/ultra.h
@@ -21,9 +21,11 @@
 #define      ULTRA_ULTRA_H
 
 #include "kernel/de/individual.h"
-#include "kernel/de/primitive.h"
 #include "kernel/de/problem.h"
 #include "kernel/de/search.h"
+#include "kernel/ga/individual.h"
+#include "kernel/ga/problem.h"
+#include "kernel/ga/search.h"
 #include "kernel/gp/individual.h"
 #include "kernel/gp/primitive/integer.h"
 #include "kernel/gp/primitive/real.h"