'For the given sets of length k, returns combined candidate sets of length k+1.'

def Ck(self, sets):

    Ck = []
    for (i, s1) in enumerate(sets):
        for (j, s2) in enumerate(sets[(i + 1):]):
            if (set(list(s1)[:(-1)]) == set(list(s2)[:(-1)])):
                Ck.append((s1 | s2))
    return Ck
