Documentation notes

p2,nokeep comes closest to saying what happens when you invoke a segment key to a node with an invalid format key. Perhaps it should be made clear that that case is covered too.
Code at the beginning of jsegment seems purposfully to consider an invalid format key to classify the node as black. This seems harmless but not a good idea.

The only principle that I see here is that it is OK to reveal the flaw in the segment for easier debugging. If you want to masquerade as something else, make a valid format key, dammit.

This suggests a new return code for a segment key to a poorly done node.

This is in contrast to invoking a start key to an invalid domain which typically falls into a black hole. I dont recall all of the reasons for doing that but one was to kill wounded objects so that they would not inadvertantly abuse their authority. Recalling the space from which an object is built may put it temporarily in a wounded state.