semid2D(f_,x_):=ds(x=>f,x,0.5);
semid2D(f_):=ds(x=>f,x,0.5);
semid2D(f1_ and f2_,x_):=ds(x=>f1,x,0.5) and ds(x=>f2,x,0.5);
semid2D(f1_ and f2_):=ds(x=>f1,x,0.5) and ds(x=>f2,x,0.5);