code: def sub_arrays(proj1, proj2):
      
      attrs = combine_metadata(proj1.attrs, proj2.attrs)
      if (attrs.get('area') is None and
              proj1.attrs.get('area') is not None and
              proj2.attrs.get('area') is not None):
          raise IncompatibleAreas
      res = proj1 - proj2
      res.attrs = attrs
      return res
docs: Substract two DataArrays and combine their attrs.