def multi_valued():
    """`True`, if objects can be indexed under different values.

    If the index can be (subscription) adapted to `IKeyedIndex`
    and `multi_valued` is `False`, then for any document, there
    is at most one key under which the document is indexed.
    This is used for `not` optimizations.

    If the index can be (subscription) adapted to `IIndexedValue`,
    then `multi_valued` specifies whether the indexed
    value should be considered atomic (`multi_valued == False`)
    or a sequence of values (`multi_valued == True`).
    """
