//[n1962_sqrt
#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();
}
//]