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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _iter_over_selections(obj, dim, values):
 """Iterate over selections of an xarray object in the provided order."""

    from .groupby import _dummy_copy

    dummy = None
    for value in values:
        try:
            obj_sel = obj.sel(**{dim: value})
        except (KeyError, IndexError):
            if dummy is None:
                dummy = _dummy_copy(obj)
            obj_sel = dummy
        yield obj_sel