...control

It is not unusual that preferences are given to some class of participants over another when a conflict arises.

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...role

[27] described four possible role constraints: role-dontcare, role-implied, role-equivalent, and role-prohibited. Actually more possibilities exist. For example, one can not take on the student role in a given course if she was once the teaching assistant of the same class. Sometimes it is up to the concensus (voting) of the admitted participants to decide whether to admit a new participant, etc.

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