struct PK_INSTANCE_sf_s
    {
    PK_ASSEMBLY_t   assembly;   --- owning assembly
    PK_TRANSF_t     transf;     --- transform (must be a rigid motion, or
                                ---     PK_ENTITY_null)
    PK_PART_t       part;       --- instanced part
    };

typedef struct PK_INSTANCE_sf_s PK_INSTANCE_sf_t;


A PK_INSTANCE_sf_t contains the definition of an INSTANCE. If the transform is
PK_ENTITY_null, then it is treated as the identity transform.