r(x,y)=sqrt(x^2+y^2)^.91
f1(x,y)=6*r(x-1,y-5)+8*r(x-3,y-5)+8*r(x-5,y-5)+21*r(x-1,y-3)+6*r(x-3,y-3)
f2(x,y)=3*r(x-5,y-3)+18*r(x-1,y-1)+8*r(x-3,y-1)+6*r(x-5,y-1)
f(x,y)=3.2+1.7*(f1(x,y)+f2(x,y))/84
x'=b*(f(x-h,y)-f(x+h,y))/(2*h)
y'=b*(f(x,y-h)-f(x,y+h))/(2*h)
par h=.01,b=5
aux z=f(x,y)
@ xp=x,yp=y,xlo=0,ylo=0,xhi=6,yhi=6
done