def count_words(pairs_list_1, pairs_list_2):
    """
    Takes two lists of pairs of the form
    [(w1, 1), ...]
    and returns a list of pairs [(w1, frequency), ...],
    where frequency is the sum of all the reported occurrences
    """
    mapping = {}
    for pl in [pairs_list_1, pairs_list_2]:
        for p in pl:
            if p[0] in mapping:
                mapping[p[0]] += p[1]
            else:
                mapping[p[0]] = p[1]
    return mapping.items()