'Generate the attack\'s symbolic graph for adversarial examples. This
method should be overriden in any child class that implements an
attack that is expressable symbolically. Otherwise, it will wrap the
numerical implementation as a symbolic operator.
:param x: The model\'s symbolic inputs.
:param **kwargs: optional parameters used by child classes.
:return: A symbolic representation of the adversarial examples.'

def generate(self, x, **kwargs):

    if (self.back == 'th'):
        raise NotImplementedError('Theano    version    not    implemented.')
    error = 'Sub-classes    must    implement    generate.'
    raise NotImplementedError(error)
