A special key string which is used to tag Shape instances so that we know they're shapes
instead of part of the shape itself.
We don't use instanceof with a Class constructor for this because that breaks when you have
multiple versions of object-shape-tester defining and consuming shapes.
A special key string which is used to tag Shape instances so that we know they're shapes instead of part of the shape itself.
We don't use
instanceofwith a Class constructor for this because that breaks when you have multiple versions of object-shape-tester defining and consuming shapes.