This is a follow-up to a discussion started about 2 months ago on 6e18b0f6 and which I did not get time to come around and finish.
This is a minimal fix: this does not answer the question of why we have such properties which we could just recompute on the fly, and does not answer why multiple documents are predicate for apparently the same purpose... It is merely to stop this unfortunate matrix API misuse before it causes problems.