A list holds at most 10 active matters: one more is 409 urn:six-minute:problem:list-full, and nothing is written. Marking a matter already on the list changes nothing, and taking one off is never refused. 404 for no such member, or for a matter this firm does not hold (unless it is on the list already and is being taken off).

Operator mount: an operator acting for the firm named in the path, as its oldest active admin, else its oldest active billing member; X-Six-Minute-Acting-As names them. 404 for no such firm, 409 urn:six-minute:problem:no-acting-member for one with neither.