+ <ptr target="(PCDM)"/> defines semantics for <ptr target="rel:first" type="property"/> and <ptr target="rel:last" type="property"/> (to point to a <ref target="ore:Proxy">Proxy</ref> of the first member), but this is confused:
+ These terms are meant to be used for horizontal linking between aggregation members, not vertical linking between aggregations and the things they aggregate.
+ Consequently, the usage of these properties with Objects is not recommended by this ontology.