[Tagging] Power generation refinement: power plant model evolution

[Alberto]

> This is where I still don't understand you: why do I need to specify 
> that
>> a feature XXX has the role XXX? Why do I need to specify, that a 
>> generator is a generator? A substation a substation? A dam a dam? A 
>> valve a valve? A weir a weir? And so on.
>>
>
> This is just because a role must be specified.
>

> Why do I have to specify a role?

Yes, roles can be omitted. In some existing types of relations members
haven't a role.
In this case we can omit them.
Viking81