Hi,

I cannot tell if I'm doing something illegitimate. I have a hard time to understand the error I'm facing:

```python import numpy as np import xarray as xr

ds = xr.Dataset( {'varA': (['x', 'time'], np.random.rand(2, 2)), 'varB': (['x', 'time'], np.random.rand(2, 2)), 'varC': (['x', 'time'], np.random.rand(2, 2))}, coords={'x': [0, 1.], 'time': [0, 1.]})

print(ds)

I want to center 'varA' and 'varB' by substracting their mean values along the time dimension:

ds[['varA', 'varB']] -= ds[['varA', 'varB']].mean(dim='time') ```

Produces the output: ``` <xarray.Dataset> Dimensions: (time: 2, x: 2) Coordinates: * x (x) float64 0.0 1.0 * time (time) float64 0.0 1.0 Data variables: varA (x, time) float64 0.7737 0.2608 0.009832 0.8498 varC (x, time) float64 0.9732 0.6492 0.5118 0.5792 varB (x, time) float64 0.04973 0.4012 0.01248 0.8716

TypeError Traceback (most recent call last) <ipython-input-36-aa87123fb25d> in <module>() 7 print(ds) 8 ----> 9 ds[['varA', 'varB']] -= ds[['varA', 'varB']].mean(dim='time')

C:\Users\poimat04\AppData\Local\Continuum\Miniconda2\envs\AxialLLRCA\lib\site-packages\xarray\core\dataset.pyc in setitem(self, key, value) 536 raise NotImplementedError('cannot yet use a dictionary as a key ' 537 'to set Dataset values') --> 538 self.update({key: value}) 539 540 def delitem(self, key):

TypeError: unhashable type: 'list' ```

Is it forbidden to assign several variables at once? I'm puzzled with this TypeError.

Thanks