A document with signatures - on erased parchment - is considered valid, though it was erased. One erasure was identifiable, as compared to repeated erasures. Note that witnesses need to attest to the status of the parchment (new/used). Also, the need to date a document with specifics that make it harder to present fraudulent accounting. For example, the reigning ruler at the time of the document being used. Plus, another interpretation of the word used to mean "ruler." Plus, the way they would handle a ruler was deposed and then returned to power.