def main():
   f = sin(x)
   a = 0
   b = 2 * pi
   N = 1000
   # consider N points in the interval [a, b]
   X = [a + (b - a) / N * k for k in range(N + 1)]
   Y = [f(x=u) for u in X]

   ll = 3
   # length of lines perpendicular to the curve
   lls = 0.3
   #  smaller subsegments

   thin_line = 2
   thick_line = 4

   #  will draw lines perpendicular to the graph of Y=f(X) at
   #  points separted by length of 'spacing'
   spacing = 0.015
   M = floor(spacing * N)

   # colors
   red = [0.867, 0.06, 0.14]
   blue = list(vector([0, 129, 205]) / QQ(256))
   green = list(vector([0, 200, 70]) / QQ(256))

   G = Graphics()
   # plot the lines
   for k in range(1, N + 1):
          p = (k - 1) * M + 2
          if p >= N:
             break
          # the normal to the curve at (X(p), Y(p))
          Normal = vector([-(Y[p + 1] - Y[p - 1]), X[p + 1] - X[p - 1]])
          Normal = Normal / norm(Normal)
          G += line([(X[p] - lls * Normal[0], Y[p] - lls * Normal[1]),
                     (X[p] + lls * Normal[0], Y[p] + lls * Normal[1])],
                    color=red,
                    thickness=0.7 * thick_line)
   # plot the curve
   G += line2d(zip(X, Y), thickness=thick_line, color=blue)
   G.axes(False)
   G.set_aspect_ratio(1)
   return G