add example circle

- demonstrates that fibers keep their identities

example/Jamfile.v2

@@ -70,6 +70,10 @@ exe segmented
     : segmented.cpp
     ;
 
+exe circle
+    : circle.cpp
+    ;
+
 #exe backtrace
 #    : backtrace.cpp
 #    ;

example/circle.cpp

@@ -0,0 +1,44 @@
+
+//          Copyright Oliver Kowalke 2016.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#include <cstdlib>
+#include <iostream>
+#include <list>
+
+#include <boost/context/fiber.hpp>
+
+namespace ctx = boost::context;
+
+int main() {
+    ctx::fiber f1, f2, f3;
+    f3 = ctx::fiber{[&](ctx::fiber && f)->ctx::fiber{
+        f2 = std::move( f);
+        for (;;) {
+            std::cout << "f3\n";
+            f2 = f1.resume();
+        }
+        return {};
+    }};
+    f2 = ctx::fiber{[&](ctx::fiber && f)->ctx::fiber{
+        f1 = std::move( f);
+        for (;;) {
+            std::cout << "f2\n";
+            f1 = f3.resume();
+        }
+        return {};
+    }};
+    f1 = ctx::fiber{[&](ctx::fiber && /*main*/)->ctx::fiber{
+        for (;;) {
+            std::cout << "f1\n";
+            f3 = f2.resume();
+        }
+        return {};
+    }};
+    f1.resume();
+
+    std::cout << "main: done" << std::endl;
+    return EXIT_SUCCESS;
+}