started advanced topics section in the docs

example/n1962/sum.cpp

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+
+#include <boost/contract.hpp>
+#include <boost/detail/lightweight_test.hpp>
+
+int sum(int count, int* array) {
+    int result;
+    auto c = boost::contract::function()
+        .precondition([&] {
+            BOOST_CONTRACT_ASSERT(count % 4 == 0);
+        })
+    ;
+
+    result = 0;
+    for(int i = 0; i < count; ++i) result += array[i];
+    return result;
+}
+
+int main() {
+    int a[4] = {1, 2, 3, 4};
+    BOOST_TEST_EQ(sum(4, a), 10);
+    return boost::report_errors();
+}
+