sequence function: frequency-class

    description:
        frequency-class(ket, sp)
        implement the frequency class equation
        see: https://en.wikipedia.org/wiki/Word_lists_by_frequency#Statistics

    the algorithm:
        drop all elements <= 0 from X
        smallest = the min coeff in X
        largest = the max coeff in X
        f = the value of e.label in X
        
        if largest <= 0:
            return 0
        if f <= 0:
            return math.floor(0.5 - math.log(smallest / largest, 2)) + 1
        return math.floor(0.5 - math.log(f / largest, 2))

    examples:

    see also:
        normed-frequency-class, map-to-topic