docs: def exists_query(expression):
      """Check that documents have a key that satisfies expression."""
      def _exists(index, expression=expression):
          """Return store key for documents that satisfy expression."""
          ev = expression() if callable(expression) else expression
          if ev:
              return [
                  store_key
                  for store_keys
                  in index.get_index().values()
                  for store_key in store_keys
              ]
          else:
              return index.get_undefined_keys()
  
      return _exists