constant listElts : (func(0, [LLChar; (Set_Set Str)]))
constant Set_sng : (func(1, [@(0); (Set_Set  @(0))]))

bind 1 a  : {a : Str | a == "director" }
bind 2 things : {v : LLChar | (listElts v == (Set_cup (Set_sng "year") 
                                               (Set_cup (Set_sng "star") 
                                                 (Set_cup (Set_sng "director") 
                                                   (Set_sng "title"))))) }

constraint:
  env [ 1; 2 ]
  lhs {v : int | true }
  rhs {v : int | Set_mem a (listElts things)} 
  id 1 tag []