code: def diffheaders(t1, t2):
      
  
      t1h = set(header(t1))
      t2h = set(header(t2))
      return t2h - t1h, t1h - t2h
docs: Return the difference between the headers of the two tables as a pair of
      sets. E.g.::
  
          >>> import petl as etl
          >>> table1 = [['foo', 'bar', 'baz'],
          ...           ['a', 1, .3]]
          >>> table2 = [['baz', 'bar', 'quux'],
          ...           ['a', 1, .3]]
          >>> add, sub = etl.diffheaders(table1, table2)
          >>> add
          {'quux'}
          >>> sub
          {'foo'}