The group.source and group.target elements may be omitted when value sets are specified for source [x] and target [x] that include codes from only a single code system, as in that case the code systems are implicitly determined and do not need to be repeated in group.source and group.target. Additionally, the minimum cardinalities for source [x] and target [x] are 0 to accomodate the need to return an "empty" concept map for $closure initiation (GF#12314).