@classmethod
def find(cls, tokens_by_line):
    """Find one instance of special syntax in the provided tokens.

        Tokens are grouped into logical lines for convenience,
        so it is easy to e.g. look at the first token of each line.
        *tokens_by_line* is a list of lists of tokenize.TokenInfo objects.

        This should return an instance of its class, pointing to the start
        position it has found, or None if it found no match.
        """
    raise NotImplementedError
