def lemmatise_series(words, postags, lemmatiser):
    """
    Create a lemma Series from token+postag Series
    """
    import nltk
    import pandas as pd
    tups = zip(words.values, postags.values)
    lemmata = []
    tag_convert = {'j': 'a'}
    for word, tag in tups:
        tag = tag_convert.get(tag[0].lower(), tag[0].lower())
        if tag in ['n', 'a', 'v', 'r']:
            lem = lemmatiser.lemmatize(word, tag)
        else:
            lem = word
        lemmata.append(lem)
    lems = pd.Series(lemmata, index=words.index)
    lems.name = 'l'
    return lems
