def lemmas(lemma,pos=None):
    """Returns all the Lemma objects of which name is `lemma` and which have `pos` as part
    of speech.

    Parameters
    ----------
    lemma : str
      Literal of the sought Lemma objects.
    pos : str, optional
      Part of speech of the sought Lemma objects. If None, matches any part of speech.
      Defaults to None
      
    Returns
    -------
    list of Lemmas
      Lists all the matched Lemmas.
    """
    lemma = lemma.lower()
    return [lemma_obj
        for synset in synsets(lemma,pos)
        for lemma_obj in synset.lemmas()
        if lemma_obj.name.lower() == lemma]