public ReplyHandler pop(Routable routable) {
        StackFrame frame = stack.pop();
        routable.setContext(frame.context);
        return frame.handler;
    }
Pop a frame from this stack. The handler part of the frame will be returned and the context part will be set on
the given reply. Invoke this method on an empty stack and terrible things will happen.

@param routable The routable that will have its context set.
@return The next handler on the stack.
messagebus/src/main/java/com/yahoo/messagebus/CallStack.java