prerequisite operation if and only if the user has already been
a member of prerequisite role. For example, a role with a
privilege to initiate a tripping command to the substation bustie
breaker is a role that has a prerequisite constraint. The
constraint is determined by the control center such as the role
must be a substation operator with a specific training and
operating experience. A user can execute this role if and only
if the user already has a role of substation operators.
Moreover, there exists many other role constraints, for
instance time constraint that defines how long the role can be
activated that may be changed from time to time according to
the live power-grid operating environment.