the Geometry type against which containment and intersection checks are carried out to determine the flow of aggregation logic
the type of any data expected to be associated with G in Feature instances
the type being aggregated to through summary mergeOp calls
This trait specifies the type of operation to be carried out but *NOT* the "zero element" which determines the beginning value for a chain of operations (like the starting element in a fold) and *NOT* the Polygon or MultiPolygon to be used during a summary. Instead, those are specified in a call to mergeOp which returns the function used in an actual summarization.