code: def diffvalues(t1, t2, f):
      
  
      t1v = set(values(t1, f))
      t2v = set(values(t2, f))
      return t2v - t1v, t1v - t2v
docs: Return the difference between the values under the given field in the two
      tables, e.g.::
  
          >>> import petl as etl
          >>> table1 = [['foo', 'bar'],
          ...           ['a', 1],
          ...           ['b', 3]]
          >>> table2 = [['bar', 'foo'],
          ...           [1, 'a'],
          ...           [3, 'c']]
          >>> add, sub = etl.diffvalues(table1, table2, 'foo')
          >>> add
          {'c'}
          >>> sub
          {'b'}