import tensorflow as tf

            alphas_all = tf.reduce_sum(alphas, 1)      # (N, L)
            alpha_reg = self.alpha_c * tf.reduce_sum((16./196 - alphas_all) ** 2)     
            loss += alpha_reg

        return loss / tf.to_float(batch_size)

    def build_sampler(self, max_len=20):
        features = self.features
