good point - we never got to actually specifying the payload part of the message spec, but it is part of the IPython implementation. I will add it to the spec for now, as it has been an assumption of every frontend implementation so far.

This is something we need to look into as a part of the spec-review we are trying to do for 2.0 (e.g. should payloads just be messages on iopub instead of inside execute_reply).