2. In Proposition 4.1 delete the second enumerated point and the sentence immediately following the proof. 3. Replace the proof of Proposition 4.1 by the following sentence:‘The proofs of all parts of this follow from the definition of≈ and by applying the standard methods for bisimulation relations.’4. Delete the third, fifth, sixth, seventh and eighth enumerated points from Proposition 4.2 and replace the proof of 4.2 with:‘Again, the proofs of these points are direct uses of the definition of simulation.’5. The sentence immediately following the proof of Proposition 4.2 should be replaced by:‘The above results tell us that we have a system that formally reconstructs the following natural properties of roles: any agent acting in one of its roles is less powerful (has fewer or equal capabilities) than the original agent; an agent E in the role E is as powerful as the agent E; if E is less powerful than F then every role of E is less powerful than the corresponding role of F.’