Return the "multiple" TypedField associated with this EntityList.

        This also lazily sets the ``_entitylist_multiplefield`` value if it
        hasn't been set yet. This is set to a tuple containing one item because
        if we set the class attribute to the TypedField, we would effectively
        add a TypedField descriptor to the class, which we don't want.

        Raises:
            AssertionError: If there is more than one multiple TypedField
                or the the TypedField type_ is not a subclass of Entity.

def _multiple_field(cls):
        """Return the "multiple" TypedField associated with this EntityList.

        This also lazily sets the ``_entitylist_multiplefield`` value if it
        hasn't been set yet. This is set to a tuple containing one item because
        if we set the class attribute to the TypedField, we would effectively
        add a TypedField descriptor to the class, which we don't want.

        Raises:
            AssertionError: If there is more than one multiple TypedField
                or the the TypedField type_ is not a subclass of Entity.
        """
        klassdict = cls.__dict__

        try:
            # Checking for cls.entitylist_multifield would return any inherited
            # values, so we check the class __dict__ explicitly.
            return klassdict["_entitylist_multifield"][0]
        except (KeyError, IndexError, TypeError):
            from . import fields
            multifield_tuple = tuple(fields.find(cls, multiple=True))
            assert len(multifield_tuple) == 1

            # Make sure that the multiple field actually has an Entity type.
            multifield = multifield_tuple[0]
            assert issubclass(multifield.type_, Entity)

            # Store aside the multiple field. We wrap it in a tuple because
            # just doing ``cls._entitylist_multifield = multifield`` would
            # assign another TypedField descriptor to this class. We don't
            # want that.
            cls._entitylist_multifield =  multifield_tuple

            # Return the multiple TypedField
            return multifield_tuple[0]