# 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);