Set()
* Create obj
  Set({})
  Set( ...) --> Set({...})
* methods
   obj_set:keys()  -- return list of key
   obj_set:set(key) -- return self
   obj_unset:(key) -- return self
* operator
 <<  : obj_set = Set() << key1 << key2 << key3 -- set
 >> : obj_set = Set() >> key1 >> key2 ; --  unset
  &  : obj_set1  & obj_set2 --> return obj_set3
  |  : obj_set1 | obj_set2 --> return obj_set3
  ==   :  obj_set1 == obj_set2 --> return bool

a=Set{'a','b'} 
b=Set{'a','b','c'}
c = a ~ b  -- {'c'}
d= b ~ a  --  {'c'}

a==b ---> 等同   c = a ~ b ; c:empty()