Define "valid"

[jyasskin]

• [ ] https://w3c.github.io/vc-data-integrity/#retrieve-verification-method says "If controllerDocument is not a valid controller document, an error MUST be raised", but the link for "controller document" doesn't give an algorithm for determining whether a document is valid.
• [ ] It also says "If verificationMethod is not a valid verification method" without an algorithm for identifying valid verification methods.