# File utilities/Flow.rb, line 34
  def draw(xmin, ymin, xnum, ynum, dx, dy)
      @flowList = GL.GenLists(1)
      GL.NewList(@flowList, GL::COMPILE)
      for i in (0 .. xnum - 1)
        for j in (0 .. ynum - 1)
          x = xmin + i * dx
          y = ymin + j * dy
          v = get(GeometricPoint.new([x,y]))
          if (v[0] != 0 || v[1] != 0)
            angle = Math.atan2(v[1],v[0]) * (180 / 3.14159)
            drawArrow(angle, x, y, 0.25)
          end
        end
      end
      GL.EndList
      GL.CallList(@flowList)
  end