This affects any AR which has a 'tree' type attribute - instrument, vocal, the new engineer AR, (and technically the orchestra attribute).

While such ARs can be valid multiple times, for different tree selections, occurring on different date periods, currently only one such AR can be added per specific entity-entity pair. This makes it impossible to say that Foo played sax on Bar between 2010-01-01 and 2010-01-01, but that Foo played piano on Bar between 2010-01-02 and 2010-01-02.