def grad_undefined(op, x_pos, x, comment=""):

    return (NullType(
        (
            "This variable is Null because the grad method for "
            "input %s (%s) of the %s op is mathematically undefined. %s"
        ) % (x_pos, x, op, comment)))()
Return an un-computable symbolic variable of type `x.type`.
dataset/ETHPy150Open rizar/attention-lvcsr/libs/Theano/theano/gradient.py/grad_undefined