def _at(self, idx):
        """Returns a view of the array sliced at `idx` in the first dim.
        This is called through ``x[idx]``.

        Parameters
        ----------
        idx : int
            index for slicing the `NDArray` in the first dim.

        Returns
        -------
        NDArray
            `NDArray` sharing the memory with the current one sliced at `idx` in the first dim.

        Examples
        --------
        >>> a = mx.nd.array([[1,2], [3, 4]])
        >>> a[1].asnumpy()
        array([ 3.,  4.], dtype=float32)
        >>> b = mx.nd.array([1, 2, 3, 4])
        >>> b[0].asnumpy()
        array([ 1.], dtype=float32)
        """
        handle = NDArrayHandle()
        if idx < 0:
            length = self.shape[0]
            idx += length
            if idx < 0:
                raise IndexError('index %d is out of bounds for axis 0 with size %d'
                                 % (idx-length, length))
        check_call(_LIB.MXNDArrayAt(
            self.handle, mx_uint(idx), ctypes.byref(handle)))
        return NDArray(handle=handle, writable=self.writable)