How to use the lineflow.apply function in lineflow

@lf.apply('ja')
def tokenize(x):
    return [START_TOKEN] + x.split() + [END_TOKEN]

@lf.apply('en')
@lf.apply('ja')
def tokenize(x):
    return [START_TOKEN] + x.split() + [END_TOKEN]

def get_indexer(key, token_to_index, unk_index):
    def indexer(token_to_index, unk_index, x):
        return [token_to_index.get(token, unk_index) for token in x]
    return lf.apply(key)(partial(indexer, token_to_index, unk_index))