docs: def ensure_hexadecimal_string(self, value, command=None):
          """
          Make sure the given value is a hexadecimal string.
  
          :param value: The value to check (a string).
          :param command: The command that produced the value (a string or :data:`None`).
          :returns: The validated hexadecimal string.
          :raises: :exc:`~exceptions.ValueError` when `value` is not a hexadecimal string.
          """
          if not HEX_PATTERN.match(value):
              msg = "Expected a hexadecimal string, got '%s' instead!"
              if command:
                  msg += " ('%s' gave unexpected output)"
                  msg %= (value, command)
              else:
                  msg %= value
              raise ValueError(msg)
          return value