sdrop seq drop all kets with coeff <= 0 from the given sequence including empty kets
sdrop (|a> . |> . 0|b> . -3 |c> . |d> . |x> + |y>) |a> . |d> . |x> + |y> sdrop (|a> + 0|b> -3|c> + |d> . |> . |x> + |y> + |z>) |a> + |d> . |x> + |y> + |z>