sigmoid: max-filter

    description:
        max-filter[t] ket
        if x <= t, return x
        else return t

    examples:
        max-filter[3] (|a> + 2|b> + 3|c> + 4|d> + 5|e>)
            |a> + 2|b> + 3|c> + 3|d> + 3|e>
            
    see also: