The declarative rule is an instance of a class derived from Rule-Declared.
* They allow for automatic processing of Property values.
* No need to be called explicitly.
* No need to run in a sequential fashion.
* The system manages re-evaluation when it detects a change.
The declarative rules in Pega are.
* Rule-Declare-Expressions.
* Rule-Declare-Index.
* Rule-Declare-Trigger.
* Rule-Declare-OnChange.
* Rule-Declare-Constraints.
