var tHdr, tMeta []byte // Zero copy bytes
	for i := 0; i < versions; i++ {
		tHdr, buf, e = msgp.ReadBytesZC(buf)
		if e != nil {
			return e
		}
		tMeta, buf, e = msgp.ReadBytesZC(buf)
		if e != nil {
			return e
		}
		if e = fn(i, tHdr, tMeta); e != nil {
			return e
		}
	}
	return nil
}

    // Take Difference of both estimates: (f(theta + eps) - f(theta - eps)).
    TF_RETURN_IF_ERROR(
        ops::Sub(ctx, fPlus.get(), fMinus.get(), f_outputs, "sub_top"));
    AbstractTensorHandlePtr fDiff(f_outputs[0]);

    // Calculate using the difference quotient definition:
    // (f(theta + eps) - f(theta - eps)) / (2 * eps).
    TF_RETURN_IF_ERROR(