holding-classes-relation.md

   +------------+      collectedBy
   |  Document  |--------------------------+
   |            |<-----------------------+ |
   +------------+      collects          | v
        ^ |                          +---------+

exemplarOf | | exemplar | Agent | | v +---------+ +--------+ holds | ^ | Item |<--------------------------+ | | |-----------------------------+ +--------+ heldBy