How to handle Material Groups in RISM

In RISM, some sources have one or more material groups (e.g. ). In the data dump, they are represented with a hasMaterialGroup triple to a material group blank node. The material group blank nodes have a rdfs:label triple indicating the name (always of the format "Group 01"), and multiple hasSummary triples, each pointing to a summary blank node. The summary blank nodes represent a property of the material group, and have an rdfs:label triple indicating the name of the property, and a rdf:value triple indicating the value of that property.

In the spirit of reducing the amount of blank nodes and dummy URIs, the current plan is to have a material group entity (of the type ), and each summary blank node gets translated into a triple, with the rdfs:label of the summary being mapped to a property, and the rdf:value of the summary used as the object of the triple.

The main issue currently is that I can't find a good property to link sources to the material groups (or the other way around), and I'm also wondering if there isn't a better way to handle the material groups.

What are your thoughts on this @dchiller ?