code: def sign(self, keys: list) -> None:
          
          if not isinstance(self.identity, Identity):
              raise MalformedDocumentError("Can not return full revocation document created from inline")
  
          self.signatures = []
          for key in keys:
              signing = base64.b64encode(key.signature(bytes(self.raw(), 'ascii')))
              self.signatures.append(signing.decode("ascii"))
docs: Sign the current document.
          Warning : current signatures will be replaced with the new ones.
  
          :param keys: List of libnacl key instances
          :return: