code: def direction(theta, phi):
      '''Return the direction vector of a cylinder defined
      by the spherical coordinates theta and phi.
      '''
      return np.array([np.cos(phi) * np.sin(theta), np.sin(phi) * np.sin(theta),
                       np.cos(theta)]f direction(theta, phi):
      '''Return the direction vector of a cylinder defined
      by the spherical coordinates theta and phi.
      '''
      return np.array([np.cos(phi) * np.sin(theta), np.sin(phi) * np.sin(theta),
                       np.cos(theta)])
docs: Return the direction vector of a cylinder defined
      by the spherical coordinates theta and phi.