1. Set Comprehension

>	a). {p:SetExp|expression-with-p}
	    Example: {p:Person|p.age > 15} represents all persons whose age is greater than 15.
	
>	b). {p:SetExp|expression-with-p|set-expression-with-p}
	    Example: {p:Person|p.age > 15|p.name} represents the names of all persons whose age is greater than 15.

2. Reverse reference

>	ClaferId.~ClaferId

3. Transitive Closure

>	ClaferId.^SubClaferId

4. InstanceOf

>	ClaferId instanceOf ClaferId	
	
>	Example:
     	
>	abstract Person
>		......
>		canDrive?
			[parent instanceOf adults]
>
>	adults:Person *
>		......
>
>	children:Person *
>		......