code: def find_missing_items(self, dtype):
          
          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)}
docs: 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