# derivative holder; ds(y_,x_,0):=y; ds(y_,x_,1):=ds(y,x); ds(y_):=ds(y,x); ds(y_ and y2_,x_,0):=y and y2; ds(y_ and y2_,x_,1):=ds(y,x) and ds(y2,x); ds(y_ and y2_):=ds(y,x) and ds(y2,x);