code: def unit_ball_L_inf(shape, precondition=True):
    
    x = tf.Variable(tf.zeros(shape))
    if precondition:
      return constrain_L_inf_precondition(x)
    else:
      return constrain_L_inf(x)
docs: A tensorflow variable tranfomed to be constrained in a L_inf unit ball.
  
    Note that this code also preconditions the gradient to go in the L_inf
    direction of steepest descent.
  
    EXPERIMENTAL: Do not use for adverserial examples if you need to be confident
    they are strong attacks. We are not yet confident in this code.