Consider an email client software. The client has one mailbox which consists of a number of diferent folder. Each folder contains a number of messages. A message cannot exist in more than one folders. A user can invoke a view on a message and in fact a user may have various views, each corresponding to one message. (a) Illustrate the appropriate mechanisms in order to identify concepts and their associations. (5) (b) Create a domain model for the email client. (13) (c) Define the term multiplicity expression and show its applicability on the domain model. (2) (d) Define the term aggregation (and its different types) and clearly illustrate its applicability to the domain model. (5)