def _calc_concat_over(datasets, dim, data_vars, coords):
    """
    Determine which dataset variables need to be concatenated in the result,
    and which can simply be taken from the first dataset.
    """
    # Return values
    concat_over = set()
    equals = {}

    if dim in datasets[0]:
        concat_over.add(dim)
    for ds in datasets:
        concat_over.update(k for k, v in ds.variables.items()
                           if dim in v.dims)

    def process_subset_opt(opt, subset):
        if isinstance(opt, str):
            if opt == 'different':
                # all nonindexes that are not the same in each dataset
                for k in getattr(datasets[0], subset):
                    if k not in concat_over:
                        # Compare the variable of all datasets vs. the one
                        # of the first dataset. Perform the minimum amount of
                        # loads in order to avoid multiple loads from disk
                        # while keeping the RAM footprint low.
                        v_lhs = datasets[0].variables[k].load()
                        # We'll need to know later on if variables are equal.
                        computed = []
                        for ds_rhs in datasets[1:]:
                            v_rhs = ds_rhs.variables[k].compute()
                            computed.append(v_rhs)
                            if not v_lhs.equals(v_rhs):
                                concat_over.add(k)
                                equals[k] = False
                                # computed variables are not to be re-computed
                                # again in the future
                                for ds, v in zip(datasets[1:], computed):
                                    ds.variables[k].data = v.data
                                break
                        else:
                            equals[k] = True

            elif opt == 'all':
                concat_over.update(set(getattr(datasets[0], subset)) -
                                   set(datasets[0].dims))
            elif opt == 'minimal':
                pass
            else:
                raise ValueError("unexpected value for %s: %s" % (subset, opt))
        else:
            invalid_vars = [k for k in opt
                            if k not in getattr(datasets[0], subset)]
            if invalid_vars:
                if subset == 'coords':
                    raise ValueError(
                        'some variables in coords are not coordinates on '
                        'the first dataset: %s' % (invalid_vars,))
                else:
                    raise ValueError(
                        'some variables in data_vars are not data variables '
                        'on the first dataset: %s' % (invalid_vars,))
            concat_over.update(opt)

    process_subset_opt(data_vars, 'data_vars')
    process_subset_opt(coords, 'coords')
    return concat_over, equals
Determine which dataset variables need to be concatenated in the result,
    and which can simply be taken from the first dataset.