Re: Guidelines to decide when a domain role needs to be explicitly modelled

Since there can be no role without a performing agent, a "specific
instances of a domain role" implies an identity distinct of the one of the
performing agent. For instance, the doctor performing the operation B must
be the same that performed the operation A.
http://caminao.wordpress.com/how-to-implement-symbolic-representations/patterns/functional-patterns/users/
Remy.