option nolet
set window -3,3,-2,2
w =0.05
a=2
b=-1
c=1
d=4
plot -3,0;3,0
plot 0,-2;0,2
for x=-3 to 3 step .25
  for y=-2 to 2 step .25
  dx = a*x+b*y
  dy = c*x+d*y
   ds=sqr(dx^2+dy^2)
 if ds>0 then plot x-w*dx/ds,y-w*dy/ds;x+w*dx/ds,y+w*dy/ds
  next y
next x
end