<element name="group" type="gml:ReferenceType" minOccurs="0">
<documentation>In a particular Time System, an Era may be a member of a group. The "group" element implements the back-pointer to the Era at the next level up in the hierarchy.
If the hierarchy is represented by describing the nested components fully in the their nested position inside "member" elements, then the parent can be easily inferred, so the group property is unnecessary.
However, if the hierarchy is represented by links carried on the "member" property elements, pointing to Eras described fully elsewhere, then it may be useful for a child (member) era to carry an explicit pointer back to its parent (group) Era.</documentation>