started advanced topics section in the docs

example/n1962/sqrt.cpp

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+
+#include <boost/contract.hpp>
+#include <boost/detail/lightweight_test.hpp>
+#include <cmath>
+
+double mysqrt(double x, double precision = 1e-6) {
+    double result;
+    auto c = boost::contract::function()
+        .precondition([&] {
+            BOOST_CONTRACT_ASSERT(x >= 0.0);
+        })
+        .postcondition([&] {
+            BOOST_CONTRACT_ASSERT(fabs(result * result - x) <= precision);
+        })
+    ;
+
+    return result = sqrt(x);
+}
+
+int main() {
+    double const precision = 1e-6;
+    BOOST_TEST(fabs(mysqrt(4.0, precision) - 2.0) <= precision);
+    return boost::report_errors();
+}
+