Ok, I know I am painfull, and I asked you to make some change, but now I think it would be nice to abstract this (and the for lines 217-220). Cause one backend could decide to store the signature on a "per cell" basis (say for progressive saving)  so for me I would put a "check and sign" and a "verify_and_trust" as `Notary` methods. This would also allow (maybe) to implement a Notary that can save (and load) partially trusted content.