final concurrency examples

futures/callbackhell.py

@@ -0,0 +1,15 @@
+def phase1(response1):
+    request2 = step1(response1)
+    fetch2(request2, phase2)
+
+
+def phase2(response2):
+    request3 = step2(response2)
+    fetch3(request3, phase3)
+
+
+def phase3(response3):
+    step3(response3)
+
+
+fetch1(request1, phase1)