docs: def comparison_operator_query(comparison_operator):
      """Generate comparison operator checking function."""
      def _comparison_operator_query(expression):
          """Apply binary operator to expression."""
          def _apply_comparison_operator(index, expression=expression):
              """Return store key for documents that satisfy expression."""
              ev = expression() if callable(expression) else expression
              return [
                  store_key
                  for value, store_keys
                  in index.get_index().items()
                  if comparison_operator(value, ev)
                  for store_key in store_keys
              ]
          return _apply_comparison_operator
      return _comparison_operator_query