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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_candidates(freq_set, k):
 """ Generate candidates for an iteration. Use this only for k >= 2. """

    single_set = {(i,) for i in set(flatten(freq_set))}

    # TO DO generating all combinations gets very slow for large documents.
    # Is there a way of doing this without exhaustively searching all combinations?
    cands = [flatten(f) for f in combinations(single_set, k)]
    return [cand for cand in cands if validate_candidate(cand, freq_set, k)]