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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 _add_to_index(index, obj):
 """Adds the given object ``obj`` to the given ``index``"""

    id_set = index.value_map.setdefault(indexed_value(index, obj), set())
    if index.unique:
        if len(id_set) > 0:
            raise UniqueConstraintError()
    id_set.add(obj.id)