We set l = 0 and u = 2. That means the number of roles to which a permission can be assigned is not more than two in the running example. In the meanwhile, we request that constraints (18) and (19) should be satisfied at the same time. The result of the two constraints is in Fig. 5(b). Constraint (19) is a simple example for prerequisite (PRE) constraints which means that U0, U1 and U2 should belong to R4.