finished all docs up to reference section

example/features/lambda.cpp

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+
+#include <boost/contract.hpp>
+#include <vector>
+#include <algorithm>
+#include <limits>
+
+int main() {
+    std::vector<int> v;
+    v.push_back(1);
+    v.push_back(2);
+    v.push_back(3);
+
+    //[lambda
+    int total = 0;
+    std::for_each(v.cbegin(), v.cend(),
+        [&total] (int const x) {
+            // Contract for a lambda function.
+            boost::contract::old_ptr<int> old_total = BOOST_CONTRACT_OLD(total);
+            boost::contract::check c = boost::contract::function()
+                .precondition([&] {
+                    BOOST_CONTRACT_ASSERT(
+                            total + x <= std::numeric_limits<int>::max());
+                })
+                .postcondition([&] {
+                    BOOST_CONTRACT_ASSERT(total == *old_total + x);
+                })
+            ;
+
+            total += x; // Lambda function body.
+        }
+    );
+    //]
+
+    assert(total == 6);
+    return 0;
+}
+