NOTES

At 9.4.3:

 * Currency was missing, added some hobby class representing a string.
 * Methodless Role Types created as PHP Interfaces.
 * Used Fully-Qualified Namespaces for method parameters.

At 9.4.4:

 * Currency grows with operations and abstract Account base class was
   added for the examples different account types.
 * Methodful Object Roles (classes) created as PHP Traits.
    >>> Traits can not be defined against the Methodless Role Type
    Interfaces. Mock as class, implement interface, stub methods and
    change class to trait, remove extends. <<<


Scala Copy:

 * Traits in PHP are compile time copy and paste only. So this is actual
   a pretty limited language feature and not a first class citizen.

    - It does not support extending classes.
    - It does not support implementing interfaces.
    - It is just members only: Constants, Property and Function definition.

 * For dynamic extension at runtime, the following two have been considered:

   1. Decorator
      (-) Can not access private members/functions

   2. Serializer
      (-) Does not work with models that do not serialize.

 * Casting via eval() for the class creation and serialize() for casting
   has been implemented.