sigmoids: invert

invert:
    description:
        invert ket
        if x approx == 0, return 0, else return 1 / x

    examples:
        invert (3|a> + 2.2|b> - 3 |c> + |d>)
            0.333333|a> + 0.454545|b> - 0.333333|c> + |d>

    see also:
        clean, ReLU