def add_MRUs(ordered_keys_and_values, return_count_only=False):
    """
        Add as many of the key/value pairs in *ordered_keys_and_values* as possible,
        without evicting any existing items.

        Returns the entries that were added, unless *return_count_only* is given,
        in which case it returns the count added instead. (This can save memory
        if the entries are not actually needed.)
        """
