Additional Exercise for Chapter 4
In their paper titled
"Towards a Logical Reconstruction of a Theory for Locally Closed Databases" 
Arieli et. all discussed the problem of representing knowledge such that
in some cases, in which be believe we have complete information, we want
to apply the closed-world assumption, while in others, we do not.
The following is a problem that illustrates this idea.
Implement its solution in ASP as an exercise.

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Exercise:
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Suppose we have a list of members of the Knowledge Representation Lab. 
We assume that we have complete information on the workplace of 
current members, but not necessarily for former or affiliate members. 

For example:

People Associated with KRlab 
______________________________
john    member
lori    member
mary    former_member
bob     affiliate
______________________________


Workplace
______________________________
john    ttu
bob     ut
______________________________


Encode this information in ASP. Define predicate employed(X) which is true
if we know that a person has a workplace, false if a person does not have
a workplace, and unknown if we don't have information on his workplace.

Example Queries and Answers:
employed(john)? -- yes
employed(lori)? -- no
employed(mary)? -- unknown
employed(bob)? -- yes