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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_child(self, collection, set, child):
 """Adds 'child' to 'collection', first checking 'set' to see if it's already present."""

        added = None
        for c in child:
            if c not in set:
                set.add(c)
                collection.append(c)
                added = 1
        if added:
            self._children_reset()