code: def _set_count(sets, n, **kwargs):
      
      sets = _set_preprocess(sets, **kwargs)
      i = as_index(_set_concatenate(sets), axis=0, base=True)
      # FIXME : this does not work for lex-keys
      return i.unique[i.count == n]
docs: return the elements which occur n times over the sequence of sets
  
      Parameters
      ----------
      sets : iterable of indexable objects
      n : int
          number of sets the element should occur in
  
      Returns
      -------
      indexable
          indexable with all elements that occured in n of the sets
  
      Notes
      -----
      used by both exclusive and intersection