Fix missing imports in examples

pep-3124.txt

@@ -179,6 +179,8 @@ annotations.  The following function definitions have identical
 effects, except for name binding side-effects (which will be described
 below)::
 
+    from overloading import when
+
     @overload
     def flatten(ob: basestring):
         yield ob
@@ -325,10 +327,11 @@ System (CLOS), or the corresponding "advice" types in AspectJ.
 Like ``@when``, all of these decorators must be passed the function to
 be overloaded, and can optionally accept a predicate as well::
 
+    from overloading import before, after
+
     def begin_transaction(db):
         print "Beginning the actual transaction"
 
-
     @before(begin_transaction)
     def check_single_access(db: SingletonDB):
         if db.inuse:
@@ -383,6 +386,8 @@ continue the invocation process.  "Around" methods are usually used
 to transform input arguments or return values, or to wrap specific
 cases with special error handling or try/finally conditions, e.g.::
 
+    from overloading import around
+
     @around(commit_transaction)
     def lock_while_committing(__proceed__, db: SingletonDB):
         with db.global_lock: