code: def add_proof(self, text, publisher_account, keeper):
          
  
          # just incase clear out the current static proof property
          self._proof = None
          self._proof = {
              'type': PROOF_TYPE,
              'created': DDO._get_timestamp(),
              'creator': publisher_account.address,
              'signatureValue': keeper.sign_hash(text, publisher_account),
          }
docs: Add a proof to the DDO, based on the public_key id/index and signed with the private key
          add a static proof to the DDO, based on one of the public keys.