docs: def _exists(field, value, document):
      """
      Ensures a document has a given field or not. ``value`` must be either True or
      False, otherwise a MalformedQueryException is raised
      """
      if value not in (True, False):
          raise MalformedQueryException("'$exists' must be supplied a boolean")
  
      if value:
          return field in document
      else:
          return field not in document