This enables "multidicts". They are a different implementation of the Python
``dict`` type, indistinguishable for the normal user (when using multidicts,
the normal implementation is not used at all).  The flexibility of multidicts
is used by a couple of other, even more advanced object implementations.

See the description in `Standard Interpreter Optimizations`_ for more details.

.. _`Standard Interpreter Optimizations`: ../interpreter-optimizations.html#multi-dicts
