def kmer_set(seq, k=4):
    """Return the set of unique k-length substrings within a the sequence/string `seq`

    Implements formula:
    C_k(s) = C(s) ∩ Σ^k
    from http://biorxiv.org/content/early/2014/08/01/007583

    >>> sorted(kmer_set('AGATAGATAGACACAGAAATGGGACCACAC'))  # doctest: +NORMALIZE_WHITESPACE, +ELLIPSIS
    ['AAAT', 'AATG', 'ACAC', 'ACAG', 'ACCA', 'AGAA', 'AGAC', 'AGAT', 'ATAG', 'ATGG', 'CACA', 'CAGA', 'CCAC', 'GAAA', ...
    """
    if isinstance(seq, basestring):
        return set(generate_kmers(seq, k))