Problem 3b.

command myline x1 y1 x2 y2
{
   set dx (x2-x1)
   set dy (y2-y1)

   repeat d 0 dx
   {
      paper 100
      set [x1 y1] 20
      set [(x1+d) (y1+d*dy/dx)] 0
      pause 1
   }
   set [x2 y2] 20
}

myline 10 10 90 90

-gary escudero