ice dynamically. 5.7.1 Changing the Existing Service by Specialization For any of the subcollaborations specified in the UML 2.x collaboration static structure specification of a service of Step 1, the subcollaboration may be specialized How to Model Dynamic Service Composition Using UML 2.x to allow for choice in dynamic composition. This is done during Step 1, or during a reiteration of Step 1. By introducing specializations as a reiteration of Step 1, the eight steps are then reiterated also. For example, for the tele-consultation case, in the subcollaboration use pcr of the call collaboration, the designer may specify a choice between three special- izations: chat, voice, or video. Depending on conditions specified in the composition policies, such as type of terminal support in the Patient's Home, one of these three specializations will be chosen during dynamic composition of the service. In Figure 5.22, the PESM diagram has been extended to include the spe- cialization of the collaboration use pcr of the call collaboration, to a choice between the collaboration use pcrl of the chat collaboration, collaboration use pcr2 of the voice collaboration, and collaboration use pcr3 of the video collaboration. This is done using the tool, by replacing the single collaboration with the three specializations, and replacing the original policy with three new policy rules, one for each specialization. We require that the specialized policy rules fulfill all the obligations of the policy rule that is being replaced. The result- ing, revised global PESM diagram for the aidedRegistration collaboration is shown in Figure 5.22. PESM AidedRegistration with call choice PDP2 Rule2s T: (Request Registration, External, Assisted caller) C: voiceOnly, assisted caller.NeedsAssistance, Reg assister.onDuty G: assisted caller.VoiceCalledTo(VCallee), PDP3_Rule1 Reg assister.VoicecalledTo(VCaller) T: ! (Answer, Reg assister, PDP2 Rulels PDP2_Rule3s assisted caller) T: (Request Registration, T: ? (Request Registration, C: True, Reg assiste