def least_common(x, rank=-1):
    """[summary]

    Args:
        x (data frame column): input DF column
        rank (int, optional): Rank, where 0 = most frequent, -1 = least frequent, 
        and other integers represent the rank. Defaults to -1.
    """
    
    cnts = x.value_counts()
    
    return cnts.index[rank]