question for self.newimg in l2_attack

[lzc]

hello, after reading your paper, I have a question, what's the meaning of this sentence:
self.newimg = tf.tanh(modifier + self.timg) * self.boxmul + self.boxplus
the input data is between [-0.5,0.5], so why this sentence is not the following:
self.newimg = tf.tanh(modifier) * self.boxmul + self.boxplus + (tf.tanh(self.timg) * self.boxmul + self.boxplus)
i don't understand tf.tanh(modifier + self.timg)