def hashable_tokens(string_to_split):
    """Split string into unique ngram tokens, sort and return as tuple,
    which is hashable.

    Args:
        string_to_split (str): string to split
    Returns:
        hashable_tokens (tuple): Hashable, standardised tuple of tokens
    """
    tokens = set(string_to_split.split(" ")) # unique tokens
    while '' in tokens:
        tokens.remove('') # ignore empty tokens
    tokens = sorted(tokens) # standardise order
    return tuple(tokens)