Extends a reference with one more component, so a producer builds a child's spelling from its
parent instead of interpolating one. Throws if the reference carries a path -- a path addresses
something with no identity of its own, so it cannot have chained descendants.
Extends a reference with one more component, so a producer builds a child's spelling from its parent instead of interpolating one. Throws if the reference carries a path -- a path addresses something with no identity of its own, so it cannot have chained descendants.