code: def addCallback(self, callback, *args, **kw):
          
          return self.addCallbacks(
              callback, _passthrough, callbackArgs=args, callbackKeywords=kw)
docs: Add a success callback that will be run in the context of an eliot
          action.
  
          @return: C{self}
          @rtype: L{DeferredContext}
  
          @raises AlreadyFinished: L{DeferredContext.addActionFinish} has been
              called. This indicates a programmer error.