def subtract(data_1, data_2):
    """ 
        Returns the difference of two arrays, data_1 - data_2. Useful for xr.apply type operations
        Author: Dougie Squire
        Date: 27/06/2018

        Parameters
        ----------
        data_1 : array_like
            The first array
        data_2 : array_like 
            The second array

        Returns
        -------
        subtraction : array_like
            The difference between data_1 and data_2

        Examples
        --------
        >>> A = xr.DataArray(np.random.normal(size=(3,2)), coords=[('x', np.arange(3)), ('y', np.arange(2))])
        >>> B = xr.DataArray(np.random.normal(size=(3,2)), coords=[('x', np.arange(3)), ('y', np.arange(2))])
        >>> doppyo.utils.subtract(A,B)
        <xarray.DataArray (x: 3, y: 2)>
        array([[-0.265376,  1.331496],
               [ 1.065077, -1.278974],
               [ 3.691209, -1.928883]])
        Coordinates:
          * x        (x) int64 0 1 2
          * y        (y) int64 0 1
    """
    return data_1 - data_2
