Inconsistency in cardinality of property sets vs material properties

This situation is valid for IfcPropertySet:

IfcWallX, IfcSlabX <- IfcRelDefinesByProperties -> IfcPropertySetX

I.e. a single property set may be shared by multiple entities.

Material properties cannot be shared.

IfcMaterial -> IfcMaterialProperties

I suspect that this is more of an "IFC5" issue, but it would be really nice to make this cardinality consistent. Right now it adds an edge case whenever anybody wants to write code that manipulates properties.

Start with the issue's comparison of IfcPropertySet and IfcMaterialProperties cardinalities in the IFC4.x specification. Determine whether the inconsistency should be resolved in IFC4.x or deferred to IFC5, then document the agreed cardinality behavior; done means the specification decision is recorded.

Written by the indexing model from the issue text.