sigmoids: clean

clean:
    description:
        clean ket
        clean the coefficients of the given superposition
        if x < 0, return 0, else return 1
        The combination: drop clean sp
        converts the given superposition into a "clean" superposition

    examples:
        -- clean a superposition:
        clean (3|a> + 2.2|b> - 3 |c> + |d>)
            |a> + |b> + 0|c> + |d>

        -- generate a "clean" superposition from an arbitrary superposition:
        drop clean (3|a> + 2.2|b> - 3 |c> + |d>)
            |a> + |b> + |d>

    see also:
        drop, ReLU, invert