SLOPE(r,x,a,b,m,y,c,d,n):=VECTOR(VECTOR([[x-(b-a)/m/(4*SQRT(1+r^2)),y-r*((b-a)~
/m/(4*SQRT(1+r^2)))],[x+(b-a)/m/(4*SQRT(1+r^2)),y+r*((b-a)/m/(4*SQRT(1+r^2)))]~
],x,a,b,(b-a)/m),y,c,d,(d-c)/n)

"Syntax is the same as Derive's DIRECTION_FIELD function"

", but SLOPE normalizes the lengths of line segments."