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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 find_missing_items(self, dtype):
 """ Find any items that are referenced in a child table but are missing in their own table. For example, a site that is listed in the samples table, but has no entry in the sites table. Parameters dtype : str table name, e.g. 'specimens' Returns --------- set of missing values """

        parent_dtype, child_dtype = self.get_parent_and_child(dtype)
        if not child_dtype in self.tables:
            return set()
        items = set(self.tables[dtype].df.index.unique())
        items_in_child_table = set(self.tables[child_dtype].df[dtype[:-1]].unique())
        return {i for i in (items_in_child_table - items) if not_null(i)}