docs: def in_query(expression):
      """Match any of the values that exist in an array specified in query."""
      def _in(index, expression=expression):
          """Return store key for documents that satisfy expression."""
          ev = expression() if callable(expression) else expression
          try:
              iter(ev)
          except TypeError:
              raise AttributeError('$in argument must be an iterable!')
          hashed_ev = [index.get_hash_for(v) for v in ev]
          store_keys = set()
  
          for value in hashed_ev:
              store_keys |= set(index.get_keys_for(value))
  
          return list(store_keys)
  
      return _in