In the statemnt: Indexes can be walked backwards by inverting their keys in a sort predicate. What do you exactly mean with "inverting their keys?
In example, if I have and index { a: 1, b:1, c:-1 } inverting the keys means having { a:-1, b:-1, c:1 } or {c:-1, b:1, a:1}?
Amazing couse by the way. Thanks!