About SERVANT : ---
Servant is used for providing some behavior to a group of classes.
Instead of defining that behavior in each class - or when we cannot factor out
this behavior in the common parent class - it is defined once in the Servant.
![alt text](./etc/servant-pattern.png "Servant")
Use the Servant pattern when
* when we want some objects to perform a common action and don't want to define this action as a method in every class.
* [Let's Modify the Objects-First Approach into Design-Patterns-First](http://edu.pecinovsky.cz/papers/2006_ITiCSE_Design_Patterns_First.pdf)